Unfortunately, the phenomenon of forging and altering (dass) the text of the ḥadīths by the hypocrites and enemies started from the early periods of Islamic history. They always intended to attack Islam in this dangerous way. However, the Prophet (PBUH) and the Imams (AS) -- and following them, our great scholars – were always aware of this danger. In order to oppose and nullify this conspiracy, they developed very effective ways to assess the chain of narrators. The scholars developed the science of Rijāl to distinguish between the trustworthy narrators and the untrustworthy ones. Moreover, in order to preserve the text of ḥadīths, they used methods such as checking the ḥadīths with the Imams (AS) and with the scholars, comparing the manuscripts, requesting permission to quote the ḥadīths, index method, etc. Therefore, the ḥadīths that have been collected in the Shiite ḥadīth collections have passed through several filters and thus constitute a rich and reliable body. Furthermore, the scholars of the sciences of Rijāl and Dirāyah still continue to assess the ḥadīths and distinguish between the forged ḥadīths and the reliable ones in order to protect and refine this most valuable source of our religion after the Qur’an.
The phenomenon of forging and altering (dass) the text of the ḥadīths by the hypocrites and the enemies – who had disguised themselves among Muslims -- already started at the time of the Prophet (PBUH). The holy Prophet (PBUH) gave many warnings against this evil phenomenon and informed the people about it. The holy Imams (AS) and, following them, the great scholars were also always aware of this danger and developed methods to oppose it.
The intellectual history of Muslims shows that they have always followed the instructions given by their religious leaders in all aspects of their intellectual endeavors, especially in the methodology for authenticating the aḥādīth of the Prophet (PBUH) and the Imams (AS). The existence of various methods for ascertaining the authenticity of the aḥadīth attests to this fact. In what follows we will briefly explain some of these methods.
First of all, most of the Shiite aḥadīth are narrated from Imam Muḥammd al-Bāqir (AS) and Imam Ja‘far al-Ṣādiq (AS) (though a few ḥadīth collections date back to an earlier period). Shaykh al-Mufīd in his Kitāb al-Irshād, Ibn Shahrāshub in his al-Manāqib, and ‘Allāmah Ṭabarsī in his A‘lām al-Warā have mentioned that four thousand trustworthy narrators (from different denominational backgrounds) have narrated aḥadīth from Imam Ṣādiq (AS). Some of these narrators succeeded to compile ḥadīth collections that later came to be called Uṣūl Arba‘ Mi’ah (“The Four Hundred Roots”). This corpus is among the earliest Shiite literature. Although only a few of these works have remained for us, the majority of the aḥadīth recorded in the Roots have been preserved in the Four Books (al-Kāfi, Man lā Yaḥḍuruhu al-Faqīh, Tahdhīb, Istibṣār).
An Aṣl (Root) is defined as a collection of the aḥadīth, taken directly by the author from the Imam (AS). If the author has not heard the Imams (AS) directly and is using another source, whose narrations are taken directly from the Imams (AS), his collection would be called a Far‘ (Branch). Thus, the occurrence of alteration or forge in these sources is very unlikely.
Although those early sources are to a great extent pure, other probabilities such as dissimulation (taqiyya) or pseudonymous authorship still exist. However, the methods of receiving ḥadīth – for instance, hearing from the author, reading the book for him, or giving permission by a Shaykh to his students for narrating ḥadīth – greatly reduce the probability of pseudonymous authorship.
Despite taking all these measures, some forged ḥadīths – made up by the Exaggerators (Ghulāt) and the Delegators (Mufawwiḍa) -- have made their way into the Shiite ḥadīth collections. It is important to remember, however, that these aḥadīth constitute only a small portion of the Shiite aḥadīth, and that several methods (explained below) have been used to discern them and purify the Shiite ḥadīth heritage.
Since there are two kinds of forging (forging both the chain of narrators and the text of a ḥadīth, and forging the text and adding to it a chain that is apparently authentic), authentication of aḥadīth takes place in two ways: (1) authentication of the text and chain of narrators, (2) textual authentication.
1) Authentication of the text and chain of narrators
The infallible Imams were the first ones who warned against the infiltration of forged ḥadīths intoShiite aḥadīth. They would mention the name of the forgers and would point to the kind of ideas and thoughts that they had forged. Imam Ja‘far al-Ṣādiq is reported to have said, “May Allah curse Mughayrah bin Sa‘īd. He would attribute lies to my father. May Allah make him feel the heat of the Fire. May Allah curse those who say about us what we do not say about ourselves. May Allah curse those who deny us being servants of Allah, who has created us, to whom is our return, and in whose hand are our forelocks.” These warnings led to rejection of the chain of narrators and texts that contained the names and the ideas of these deviant groups, and the Imams’ companions and the Shiite narrators were able to carefully delete such deviations to a great extent.
Purification of the aḥadīth continued with the authors of the Four Books (al-Kāfi, Man lā Yaḥḍuruh al-Faqīh, Tahdhīb, Istibṣār). These pious and knowledgeable scholars testified that they had collected the aḥadīth that they believed to be authoritative (ḥujjah) between them and their Lord, i.e., the ones they considered to be authentic (Ṣaḥīḥ). And we know that the early scholars would use the term Ṣaḥiḥ for the ḥadīth they were confident had actually been said by the Infallibles. This confidence was a result of scrutinizing the text and the chain of narrators of that Ḥadīth.
At any rate, before the compilation of the four Rijāl books (i.e., al-Rijāl by al-Kashshī, al-Fihrist by al-Najāshī,and al-Rijāl and al-Fihrist by al-Ṭūsī), our scholars had several Rijāl works, such as the works by ‘Abd Allah b. Jabala al-Kinānī (d. 219 AH), Muḥammad b. ‘Isā al-Yaqṭīnī, Ḥasan b. Maḥbūb (d. 224 AH), Ḥasan b. ‘Alī al-Faḍḍāl (d. 224 AH), and others.  This means that following the instructions of Ahlulbayt (AS), our Ḥadīth narrators were very careful about the aḥadīth they collected; they would not collect just any ḥadīth they found, but only the ones that met a set of critical standards. After that, the four Rijāl books were compiled. These books contributed greatly to knowing the narrators and the Ḥadīth collections and to authenticating the attribution of those collections to their alleged authors.
B) Textual authentication
To purify the texts of the aḥadīth from any kind of intentional or unintentional alteration and forge the following methods were used:
1) Presenting the aḥadīth to the Infallible Imams (AS) and the scholars:
One of the ways used especially by the companions of Imam Riḍā (AS) and the later Imams to check the authenticity of the aḥadīth was checking them with the Imams (AS) or their renowned and reliable companions. This method, called in our aḥadīth “‘arḍ al-ḥadīth” (presenting the ḥadīth [to the Imams or the scholars]), shows itself mostly in the first phase of collecting the aḥadīth, i.e. "the period of presence". The method of “presenting the ḥadīth” was a constant procedure that always accompanied the process of narration. In the early ḥadīth literature, instances of presenting the aḥadīth to Imam ‘Alī (AS), Imam Ḥasan (AS), Imam Ḥusayn (AS), Imam Sajjād (AS), Imam Bāqir (AS), and Imam Ṣādiq (AS) are found. Presenting several ḥadīth collections to Imam Ṣādiq (AS) indicates an increase in the application of this method in the Imam’s period; a period in which the identity of Shiism was consolidated and the Ja‘farī Shiism separated its way from the Zaydiyya and other sects. The companions and narrators who were educated in this school became measures for authenticating what was attributed to Shiism and to the Imams, and they in turn secured the Shiite ḥadīth heritage by presenting their ḥadīth collections to the Imams (AS).
One of the most important deviant trends of this period is the trend of Exaggeration (ghuluww). The Exaggerators were power-seekers who elevated the Imams (AS) to a divine status, and then claimed to be their deputies, in order to achieve their evil goals. The Infallible Imams (AS) were aware of these deviant groups and their danger, and they would inform others of the threat they posed. Instances of such treatment can be found in the main Rijāl books, especially Rijāl al-Kashshī, in the case of Exaggerators like Abū al-Khaṭṭāb and Yunus b. Ẓabyān.
It is reported that Yunus b. ‘Abd al-Raḥmān presented several Shiite ḥadīth collections to Imam Riḍā (AS), and the Imam (AS) rejected many of the aḥadīth contained in one of them.
It is also reported that the books of Banū Faḍḍāl were presented to Imam Ḥasan al-‘Askarī (AS). In this case, the Imam (AS) verified the aḥadīth recorded in them.
The method of presenting the aḥadīth caused naivety to disappear, and the ludicrous forgers to be driven away from the Shiite ḥadīth enterprise. The frightful suspicion that the Imam (AS) would reject a ḥadīth was enough to bring down the hypocrites who had infiltrated into the lines of the Shiites from the position of narrating aḥadīth and ban them from its pure territories – though they were not completely removed from it.
2) Comparing the copies
From an early period, the scholars of Islam and the companions of the Prophet (PBUH) and of the Imams (AS) used to check their own Ḥadīth collections with the original or authentic sources. This was an effort for purifying and correcting the intentional and unintentional alterations and forges.
3) Permission for narration
The Ḥadīth scholars have regarded “permission for narration” as one of the most important ways of receiving Ḥadīth and a supporting evidence for authenticity in the process of narration. They ask their teachers for permission to narrate the aḥadīth that they have learned from them. The teachers then give an oral or written permission, often containing the name of their own teachers and the titles of their works, to their qualified and trustworthy students. For instance, Aḥmad b. Idrīs al-Ash‘arī al-Qummī was a famous faqīh and ḥadīth scholar and one of the prominent figures of the third century in Qum, who had the honor of visiting Imam Ḥasan al-‘Askarī (AS). He studied different disciplines of knowledge with great Imami scholars of his time, such as Aḥmad b. Isḥāq al-Qummī (the representative of the twelfth Imam – AS), and was able to write precious compilations in fiqh and ḥadīth. He has narrated many ḥadīths from the holy Imams (AS). He was the teacher of great Shiite scholars and gave them the permission for narration. One of his most famous and important students is Muḥammd b. Ya‘qūb al-Kulaynī, who produced the most reliable Shiite ḥadīth collection, i.e. al-Kāfī, in which he directly narrates from Ibn Idrīs.
An important benefit of the tradition of “permission for narration” is preserving the connectedness of the chains of narrators to the infallible Imams (AS). In the later periods, the teachers usually mention the chain of narrators only up to one of the great scholars such as Mulla Muḥammad Taqī al-Majlisī, al-Shahīd al-Awwal, al-‘Allāma al-Ḥillī, or al-Shaykh al-Ṭūsī, because the chain of narrators from these scholars to the infallible Imams (AS) is well-known.
4) The Indexical Method
This is a method usually applied by the early scholars of ḥadīth, who would assess the narrations based on a certain set of criteria. For instance, the scholars of Qum were very sensitive about the Exaggeration currents; they would reject any narration that would in any way promote Exaggeration thoughts. Because of this, Ibn Walīd, for instance, while narrating all of the works written by al-Ṣaffār, does not narrate the latter’s Baṣā’ir al-Darajāt. Or, for instance, although he approves of the narrators mentioned in the book Nawādir al-Ḥikma, he excludes some them, not because they were unreliable, but the narration narrated by them in that book was not acceptable to him. Although each scholar may have a different way of using this method based on his respective opinion, it is a method applied for textual purification of the narrations.
5) The agreement of the ḥadīth's text with the clear teachings of the Qur'an
Another method is checking the aḥadīth with the Qur'an. The Prophet (PBUH) and the holy Imams (AS) have been repeatedly reported to have said, "Whenever a ḥadīth reaches you [allegedly] from us, check it with the book of Allah. Hold to the one that was in agreement with the Qur'an, and leave the one that was opposing the Qur'an, or refer it to us."
Moreover, Allah had appointed guards for the precious Prophetic heritage. These guards were the points of reference for the truth-seeking believers. One instance is Sulaym b. Qays al-Hilālī; he had discovered some of the origins of the confusions, discrepancies, and forges that the Prophetic ḥadīth suffered from, and he came to the conclusion that the only secure way of transmission for the Prophetic Ḥadīth was through the Ahlulbayt (AS).
In sum, with the painstaking efforts of the Imams (AS) and their companions, we have received a ḥadīth heritage which is rich and to a great extent authentic. However, the scholars do not regard further investigation about the text and the chain of narrators of a ḥadīth dispensable; they continue to search for additional evidence and criteria for distinguishing between the authentic and the inauthentic ḥadīths. After these researches and based on such criteria and evidences, the scholars may reject a Ḥadīth which was previously regarded authentic. Examples include when a Ḥadīth (1) is clearly opposing the Qur'an, (2) is against the principles of our madhhab, (3) is rejected by the Scholars of Imāmiyya throughout Their history, (4) is said because of taqiyya, (5) has signs of forge. These and other sophisticated and technical methods are used today by the scholars to accept or reject a Ḥadīth.
 Mudīr Shānichī, Kaẓim, Tārīkh-i Ḥadīth, p. 91.
 Ḥarīrī, Muḥammad Yūsuf, Farhang-i Iṣṭilāḥāt-i Ḥadīth, p. 11.
 Māmqānī, Talkhīṣ Miqbās al-Hidāyah, p. 156.
 Ṣadūq, Man lā Yaḥḍuruh al-Faqīh, vol. 1, p.3, Intishārāt Jāmi‘i Mudarrisīn.
 Sayfī Māzandarānī, ‘Alī Akbar, Miqyās al-Riwāyah fī ‘Ilm al-Dirāyah, p. 44.
 Mudīr Shānichī, Kaẓim, Tārīkh-i Ḥadīth, p. 55-6.
 Al-Kashshī, al-Rijāl, vol.2, p. 692-700; Sulaym b. Qays al-Hilālī, Kitāb Sulaym b. Qays, vol.2, p. 558, 562.
 Ibn Sa‘d, Ṭabaqāt, vol. 3, p. 26, Quoting Musnad Imām al-Mujtabā p.535, ḥadīth #36.
 Sulaym b. Qays al-Hilālī, Kitāb Sulaym b. Qays, vol.2, p. 628; Al-Mustadrak ‘alā al-Ṣaḥīḥayn, vol.3, p.187, ḥadīth #4798; Da‘ā’im al-Islām, vol.1, p.142; al-Ja‘fariyyāt, p.5, ḥadīth #42; Tafsīr al-‘Ayyāshī, vol.1, p.157, ḥadīth #530.
 Al-Kāfī, vol.8, p.15, ḥadīth # 2; Sulaym b. Qays al-Hilālī, Kitāb Sulaym b. Qays, vol.2, p. 559; Al-Kashshī, al-Rijāl, p.104, #167.
 Al-Shaykh al-Ṭūsī, al-Fihrist, p. 176; Ma‘ānī al-Akhbār, p. 382, ḥadīth #12.
 Al-Kashshī, al-Rijāl, p.364, 546, 363, 292, 291; Ma‘ānī al-Akhbār, p. 181, ḥadīth #1; Biḥār al-Anwār, vol.26, p.140, ḥadīth #12.
 Al-Kashshī, al-Rijāl, p.224.
 Subḥānī, Ja‘far, Kulliyyāt fī ‘Ilm al-Rijāl, p.278.
 Al-Kashshī, al-Rijāl, p.251, (the section on Muḥammad b. al-Ḥasan al-Ṣaffār), Dāvarī Press.
 Al-Najāshī, al-Rijāl, p.245, (the section on Muḥammad b. Aḥmad b. Yaḥyā), Dāvarī Press.
 Tahdhīb al-Aḥkām, vol.7, p.275:
روی عن النبی ص و عن الأئمة ع أنهم قالوا إذا جاءکم منا حدیث فاعرضوه على کتاب الله فما وافق کتاب الله فخذوه وما خالفه فاطرحوه أو ردوه علینا
 Taken from Ḥadīth Sciences (‘Ulūm Ḥadīth) journal, number 6, by ‘Abd al-Hādī Mas‘ūdī.