There is not a tradition in Shia’s reliable sources in which Prophet Muhammad (S) may have said that fasting on the day of Ashura is recommended but there are narrations concerning the seerah (lifestyle) of the Prophet (S) in which it is said that he observed fast on this day. For example, it has been reported from Imam Reza (A.S.) that he said: «صَامَ رَسُولُ اللَّهِ (ص) يَوْمَ عَاشُورَاء» (The Messenger of Allah (S) observed fast on the Day of Ashura.)
Obviously, one cannot say, with reliance on the above narration that the Prophet (S) considered fasting on the day of Ashura as recommended because in addition to the weakness of the chain of transmission of such narrations, they just make reference to the conduct of the Prophet (S) and there is no mention of the desirability or abominableness of fasting.
Indeed, it is inferred from narrations existing in both Shia and Sunni sources that Ashura had been revered and regarded as sanctimonious from the time of Prophet Moses (A.S.) until the prophethood of the Messenger of Allah and from the Prophethood until the fast of the month of Ramadhan became obligatory. Thus, Ashura was considered to be one of the days of Allah. However, following the event of Ashura and the Bani Umayyad’s exploitation or misuse from the narrations concerning fasting and the commemoration of Ashura, different narrations have been transmitted in this regard some indicating that fasting on this day is desirable and recommended, others indicating that it is abominable and a third group of verses even imply prohibition of fasting.
Hence, this day should not be commemorated as a happy day lest the memory of the martyrdom of Imam Hussain (A.S.) and his kith and kin may not go into oblivion. The martyrdom of Imam Ali (A.S.) also took place on the night of Qadr whose fast is obligatory. There cannot be any problem in fasting on the Day of Ashura, per se but since fasting on this day had been turned into a propagation tool in the hand of Banu Umayyad, most of the of jurisprudents have therefore considered fasting on this day as abominable. That is to say, fasting on this day is undesirable. Some other scholars have said that fasting is not permissible on the day of Ashura as a measure of obligatory precaution.
The question consists of two parts and each part has to be dealt with separately:
A) The alleged saying from the Prophet (S) about fasting on Ashura
There is not a tradition in Shia’s reliable sources in which Prophet Muhammad (S) may have said that fasting on the day of Ashura is recommended but there are narrations concerning the seerah (lifestyle) of the Prophet (S) in which it is said that he observed fast on this day. For example, it has been reported from Imam Reza (A.S.) that he said: «صَامَ رَسُولُ اللَّهِ(ص) يَوْمَ عَاشُورَاء» (The Messenger of Allah (S) observed fast on the Day of Ashura.)
Indeed, in some traditions it has been narrated that the Commander of the Faithful, Ali (A.S.) said: “Fast on the ninth and the tenth of Muharram because it is the kaffara (penalty) of the sins (you have committed) in a year.”
Nonetheless, it cannot be alleged that there are sayings in Shia’s reliable sources in which the Prophet (S) may have said that fasting on the day of Ashura is recommended. That is because some of the mentioned narrations are weak. In addition to the weakness of the chain of transmission of such narrations, they just make reference to the conduct of the Prophet (S) and there is no mention in his saying as to the desirability of fasting on such a day. Of course, the Sunni sources do contain traditions from the Prophet (S) regarding the desirability of fasting on the day of Ashura.
B) Fasting on the Day of Ashura as Viewed by Shia Jurisprudents
What is inferred from narrations existing in both Shia and Sunni sources is that Ashura had been revered and regarded as sacred from the time of Prophet Moses (A.S.) until the prophethood of the Messenger of Allah and from the Prophethood until the fast of the month of Ramadhan became obligatory.
However, following the event of Ashura, the Banu Umayyad rulers misused the narrations about fasting and commemoration of the day of Ashura. They propagated something which was totally opposed to the religion and the Islamic school. They said that since Hussain bin Ali had been killed on that day and his death and the death of his comrades and kins was a good act, therefore, this day is a blessed day and it is for this reason that Muslims must fast on this day. That was why they fabricated many traditions in a bid to change and misrepresent the event of Ashura and to declare it as a day of festivity and happiness in which they should observe fast out of thankfulness to God.
Karajaki (r.a.) one of the fifth century Shia scholars writes in his “Al-Ta’jjob min Aghlat al-‘Ammah fi Mas’alat al-Imamiyah” that some people who claim to have love of the Ahlul-Bayt (A.S.) in their hearts celebrate the day of Ashura saying that this is the day of the repentance of Prophet Adam (A.S.) whereas they do not sympathize with the Holy Prophet (S) who lost his beloved grandson.
Therefore, the desirability of fasting on the day of Ashura is a totally misleading and deceptive ruling that has found a place among a group of so-called Muslims. Imam Hussain (A.S.) makes reference to the same point in his Ziarat Ashura in which he says: “This day is a day of rejoicing for the Bani Umayyah.”
It is from the same date that the fast of the day of Ashura could be observed for two justifications:
1. The fast of this day has been observed either as a recommended or obligatory fast because this day has been considered among the ‘day of Allah’.
2. The fast of this day has been observed out of thankfulness to Allah for the death of Imam Hussain (A.S.) at the hand of Yazid. It was from this time onward that the infallible Imams (A.S.) combated misleading thought by enlightening people’s mind and forbidding them from fasting on this day. Zurarah narrates from Imam Sadiq and Imam Baqir, peace be upon them, that they said: “Do not fast on the day of Ashura…”
In any case, the narrations existing in this regard are different. Some of the narrations indicate the desirability of fasting on this day saying that the fasting on the ninth and the tenth of Muharram is the kaffara (penalty) of the sins (you have committed) in a year. According to the same category of narrations, the Holy Prophet (S) observed fast on the same day. Some other narration imply abominableness or even prohibition of fasting on this day e.g. the narration which says that fasting on this day is a practice of Ziad family and Ibn Marjanah.
Now that there are different narrations in this regard, the question is: How is possible to reconcile between these two opposing sets of narrations. This is how to combine between them:
1. Fasting takes place into ways: One, for rejoicing and showing happiness and another for seeking proximity to God. The narrations which forbid fasting on the day of Ashura refer to the first type of fasting and the narrations which consider fasting on this day refer to the second type of fasting.
2. It is likely that the narrations which indicate permissibility of fasting on this day may have been abrogated. This probability is certified and affirmed by some reports according to which fasting on the day of Ashura had been obligatory in the beginning but then it had been repealed. It has been said that fasting on the day of Ashura had been obligatory during the first hegira year and it was abrogated with the coming of the fast of Ramadhan in the second year. For example, Imam Muhammad Baqir (A.S.) has been reported as having said:
Muhammad bin Yahya has narrated Ahmad ibn Muhammad and Muhammad ibn al-Hussain both of them from Muhammad ibn Ismail ibn Bazi’ who from Mansur ibn Yunus, from Abu Jarud who has said the following: ‘I heard Abu Ja’far (A.S.) say: ‘Allah has commanded people to fulfill five obligations. They have undertaken the responsibility for four of these obligations but they have ignored one.’
The Narrator has said: ‘I asked the Imam (A.S.), ‘May Allah keep my soul in service for your cause, will you please, describe them for me?’
The Imam said: ‘The prayer is one of such obligations. People did not know how to perform the prayer. Jibril descended and said: ‘O Muhammad (S), explain to them the time for the Prayer.’ Then the Command to pay Zakat came. Jabril then said: ‘O Muhammad (S), explain to them how to pay Zakat as you have explained to them about the prayer.’ Then the Command to fast came. The Messenger of Allah would send information to the nearby towns and would ask people to fast on the Day of Ashura and people would fast on that day.
Then the (Command to fast during the) month of Ramadhan came between the month of Sha’ban and Shawwal (and fasting on the day of Ashura was abrogated).’
3. The narrations which indicate the significance of fasting on the day of Ashura are treated as narrations implying Taqiyah (precautionary dissimulation).
4. What late Abul Fazl Tehrani has considered probable in Shifaa al-Sodoor by telling that the narrations which imply permissibility of fasting on this day can be taken as narrations indicating partial imsak (restraining from eating and drinking). On the other hand, the narrations which forbid fasting on this day can be taken as narrations which indicate prohibition of complete imsak. Conclusively, fasting on the Day of Ashura amounts to disbelief or is either haram or makrooh (abominable) and there is never any likelihood of the fast being recommended in the religion and Fiqh of Ahlul-Bayt (A.S).
It has been said in the explanation of the above passage that if a person observes fast on this day because of a special feature, it is bid’ah and forbidden and if he observes fast on Ashura for tabarruk (blessing), it amounts to disbelief and going out of the religion and if he observes fast because there is virtue in fasting in an absolute way, it is makrooh (undesirable) and deficient in reward because it has a formal resemblance to Umayyad’s practice.
Therefore, the catastrophic and tragic aspect of the day of Ashura is more outstanding than its positive aspect considering the fact that the Banu Umayyad dynasty left no stone unturned to wipe out the memory of the event of Ashura. For this reason, contemporary scholars have stated that fasting on the day of Ashura is abominable. Some have said that it is mustahab (recommended) to avoid eating and drinking until midday of Ashura without making any niyat of fasting. As some jurisprudents have said, fasting on the day of Ashura is not permissible, as an obligatory precaution.
 - Shaykh Tusi, Al-Istibsar fi-ma Ikhtalafa min-al Akhbar, vol.2, p. 134, Dar al-Kutub al-Islamiyah, Tehran, 1st edition, 1390 A.H.; Shaykh Hurr Amili, Wasail al-Shi’ah, vol.10, pg. 457, Aalulbayt (A.S.) Institute, Qom, first edition, 1409 A.H.
 - Al-Istibsar, vol.2, pg. 134:
: «عَلِيُّ بْنُ الْحَسَنِ بْنِ فَضَّالٍ عَنْ هَارُونَ بْنِ مُسْلِمٍ عَنْ مَسْعَدَةَ بْنِ صَدَقَةَ عَنْ أَبِي عَبْدِ اللَّهِ(ع) عَنْ أَبِيهِ أَنَّ عَلِيّاً ع قَالَ: "صُومُوا الْعَاشُورَاءَ التَّاسِعَ وَ الْعَاشِرَ فَإِنَّهُ يُكَفِّرُ ذُنُوبَ سَنَةٍ».
 - Qazvini, Hafiz Abi Abdillah Muhammad bin Yazid, Sunan Ibn Majah, reasearched by Abdul Baqi, Muhammad Fu’ad, vol.1, p. 552, Chap. Fasting on the Day of Ashura, Darul Fikr, Beirut, first edition, (date not mentioned); Abdullah bin Abdur Rahman, Sunan al-Darami, vol.2, p. 22, Chap. Al-Bari, Damascus, 1349 A.H.; Qushairi Neishabouri, Muslim bin Hajjaj, Sahih Muslim, vol.3, p. 150, Dar al-Fikr, Beirut, (date not mentioned).
 -For example, about 30 traditions have been mentioned in Sahih Muslim.
 - Vide: Shaykh Tusi, Tahzib al-Ahkaam, researched and edited by Musavi, Hasan, vol.4, p. 301, Dar al-Kutub al-Islamiyah, Tehran, 4rth edition, 1407 A.H.
 -Karajaki, Muhammad bin Ali, “Al-Ta’jjob min Aghlat al-‘Ammah fi Mas’alat al-Imamiyah”, p. 115, Dar al-Ghadir, Qom, first edition, 1421 A.H.
 - «و هذا یوم تبرّکت به بنو امیّة».
 -Al-Isibsar, vol.2, p. 134; Kulayni, Muhammad bin Ya’qub, researched and edited by Ghaffari, Ali Akbar, Akhundi, Muhammad, vol.4, p. 146, Dar al-Kutub al-Islamiyah, Tehran, fourth edition, 1407 A.H.
 - Tahzib al-Ahkaam, vol.4, pg. 301 and 302.
 - See: Al-Kafi, vol.4, p. 146.
 - Al-Kafi, vol.1, p. 290:
«مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ وَ مُحَمَّدِ بْنِ الْحُسَيْنِ جَمِيعاً عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ بْنِ بَزِيعٍ عَنْ مَنْصُورِ بْنِ يُونُسَ عَنْ أَبِي الْجَارُودِ عَنْ أَبِي جَعْفَرٍ (ع) قَالَ سَمِعْتُ أَبَا جَعْفَرٍ (ع) يَقُولُ فَرَضَ اللَّهُ عَزَّ وَ جَلَّ عَلَى الْعِبَادِ خَمْساً أَخَذُوا أَرْبَعاً وَ تَرَكُوا وَاحِداً قُلْتُ أَ تُسَمِّيهِنَّ لِي جُعِلْتُ فِدَاكَ فَقَالَ الصَّلَاةُ وَ كَانَ النَّاسُ لَا يَدْرُونَ كَيْفَ يُصَلُّونَ فَنَزَلَ جَبْرَئِيلُ ع فَقَالَ يَا مُحَمَّدُ أَخْبِرْهُمْ بِمَوَاقِيتِ صَلَاتِهِمْ ثُمَّ نَزَلَتِ الزَّكَاةُ فَقَالَ يَا مُحَمَّدُ أَخْبِرْهُمْ مِنْ زَكَاتِهِمْ مَا أَخْبَرْتَهُمْ مِنْ صَلَاتِهِمْ ثُمَّ نَزَلَ الصَّوْمُ فَكَانَ رَسُولُ اللَّهِ ص إِذَا كَانَ يَوْمُ عَاشُورَاءَ بَعَثَ إِلَى مَا حَوْلَهُ مِنَ الْقُرَى فَصَامُوا ذَلِكَ الْيَوْمَ فَنَزَلَ شَهْرُ رَمَضَانَ بَيْنَ شَعْبَانَ وَ شَوَّال...»؛
Majlisi, Muhammad Baqir, Mir’at al-Uqool fi Sharh Akhbar Aal al-Rasool, researched and edited by Rasuli Mahallati, Hashim, vol.3, p. 259, Dar al-Kutub al-Islamiyah, Tehran, second edition, 1404 A.H.
 - Tabasi, Najmuddin, Fasting on the Day of Ashura: a prophetic tradition or an Amawid innovation, translated by Dadash Zadeh, Mustafa, p. 43, (date not mentioned), Qom, first edition, 1429 A.H.
 - Vide: Tehrani, Mirza Abul Fazl, Shifa al-Sodoor fi Sharh Ziarat al-Ashur, p. 392-393, Murtazavi Publications, Tehran, second edition, 1383 A.H.
 - Vide: Thaqafi Tehrani, Hasan, Complete Translation and Annotation of Ziarat Ashura (Summary and a Fresh Opinions and Modification of Shifa al-Sodoor fi Sharh Ziarat Ashura), p. 420, Had Publication, first edition, 1385 (2006).
 - Imam Khomeini, Tawzih al-Masail (with annotation), researched and edited by Bani Hashemi Khomeini, Sayyid Muhammad Hussein, vol.1, p. 967, issue 1747, Islamic Publications Office, Qom, eighth edition, 1424 A.H.
 - Wahid Khurasani, Hussain, Tawzih al-Masail, p. 334, issue No.1755, Madrasa Imam Baqir (A.S.), Qom, Ninth edition, 1428 A.H.