Shawahid al-Tanzil is one of the important books that has drawn the attention of Muslim scholars and intellectuals all over the Islamic world. There is a difference among the scholars as to the religion of the writer. Some consider him as Shia; others believe that he was the follower of Abu Hanifa. The author's motivation for writing this book had been to defend the sacred personality of Imam Ali (a.s.) and the Quranic verses about him that have been doubted by some fanatics. Since the writer of Shawahid al-Tanzil is one of the Sunni narrators and lived in a predominantly Sunni populated area, Sunni seminaries and worked in such an environment, he has made profuse use of Sunni narrative sources, books and narrators. It is for the same reason that this book is unique and distinct among other books in this respect.
The answer to your question will be given in two parts:
1. The Life of the Author (Hakim Haskani):
A short bibliographical description of the life of the author of Shawahid al-Tanzil published in a book is as under:
“Al-Haskani, the judge, the Muhadith, Abu al-Qasim Ubaidllah bin Abdullah bin Ahmad bin Muhammad bin Haskan al-Qurashi al-Ameri al-Neishaburi al-Hanafi al-Hakim, known as ibn al-Heda, Hafiz, a preserved Sheikh, very careful with the knowledge of hadith is from Abdullah bin Amir bin Kuraiz descent, who conquered Khurasan during Uthman’s time."
There is no clear information about his birth and death. Dhahabi writes in his Tadkirat al-Hufaz (Biography of Memorizers) that he lived until 470 after Hegira and died after this year. That is why he is considered to be one of the Muslim scholars of the fifth century of the Islamic calendar. He was a Sunni scholar and Dhahabi explicitly states in Tadkirat al-Hufaz that he was a follower of Abu Hanifa. For this reason, he has introduced him as "Abul Qasim Ubaidullah al-Hanafi".
In this connection, Ya'qub Ja'far says: The author of the book is Abul Qasim Hakim Haskani; he is one of the great Sunni traditionists and Hufaz who came from Neishabour and, as his student Abdul Ghafir says, he is the follower of the Hanafi religion. In view of some of the works he has written about the virtues and merits of the Commander of the Faithful, it can be said that he had a strong tendency towards Shi'ism. Some of the works written by him on the merits of the Commander of the Faithful, Ali (a.s) are "A Treatise on the Virtues of the Commander of the Faithful, Ali (a.s.),", "A Treatise on correcting the (hadith of the) return of the sun", and the book in question "Shahwahid al-Tanzil". For this reason Dhahab says:
“I found some majalis (discourses) of him that underscore his Shiasm and his experience in hadith." That treatise is the "Treatise on correcting the (hadith of the) return of the sun".
Owing to Haskani's Shawahid al-Tanzil and the two other treatises he has written, Allamah Aqa Buzurg Tehrani has counted him among Shia authors. He introduces him and Shawahid al-Tanzil in his Al-Zari'ah Ela Tasanif al-Shi'ah, a book on Shia authors and their works.
It can be deduced from the above that Haskani was a Hanafi Sunni in the beginning but as he gained more knowledge and an unbiased expertise in hadith and of traditions, he was guided in the right path. Thus, he accepted the Shia religion and was included amongst Shia scholars.
Haskan had been the name of a village in one of the districts of Neishabour. That is why some people who are from Neishabour and who were born there are called Haskani.
Hakim Haskani led a long life and his chain (isnaad) of the traditions was highly (preserved). He had many teachers and students whom we cannot name in this article for the sake of brevity. He studied Fiqh (jurisprudence) with Abul 'Alaa Sa'ed bin Muhammad. He practiced Arabic literature so much that he attained the highest level of knowledge in it. Of course, he was an expert in hadith and science of biography. He authored as many as one hundred books the most important of which, perhaps, is Shawahid al-Tanzil.
2. Shawahid al-Tanzil
A) The Motivation for Compilation of This Book
In the preface to this book, the author writes as such: "Some scholars who are ruling over the people and are among the leaders of Keramiyah sect have established scientific and discussion circles with many religious students gathering round them. Unfortunately, this so-called scholar (Muhammad bin Keram) has questioned the Alavid sanctity by his demagoguery and incorrect words. His babbling has reached its peak and he has said: "No exegete of the Quran has claimed that Sura Hal Ata (Dahr, Insan) has been revealed about Imam Ali (a.s) and his family." He has gone further beyond this and has said that no verse has been revealed about the Ahlul-Bait (a.s.). I was amazed to hear this and to see his discourtesy and rudeness. Thus, I considered his saying as a great lie and expected other scholars to raise their voice and criticism but it is a matter of great pity and regret that no one voiced anger and protest. The only person who took an action was my great teacher Abul 'Alaa Sā'eid bin Muhammad.
He reproached some of his students who were present in that circle and he sufficed to it and ignored their prohibitions despite being a man who attached importance to amr bil-ma'ruf and nahy anil munkar (enjoining good and forbidding bad). This made me feel a duty and make my mind firm to provide an elaborate answer to this misgiving and to gather the answers in a single collection. That was why I embarked on collecting the traditions concerning the verses that have been revealed about Imam Ali (a.s.) and his family. I referred to different sources and I gathered in this book everything that was available in this regard.
Of course, the author says at the end that he has accomplished this scientific work with haste and he did not succeed to collect the key to traditions concerning the merits of the Commander of the Faithful, Ali (a.s.). In fact, he has rendered what has been available to him and ignored the sources which he did not have an access to.
B) The Author's Method of Compilation
In the introduction to his book, Haskani gives the reason for the name and title of the book and his motivation behind writing it. Thereupon, he has arranged six chapters on different themes as under:
The first chapter deals with the statements and sayings of ancient scholars concerning Imam Ali's merits and virtues. Some of those scholars are Abdullah bin Abbas, Ahmad bin Hanbal, Abdullah bin Umar and Aisha.
The second chapter deals with Imam Ali's background in recitation and memorization of the Holy Quran. The author relates many narratives in this regard.
The third chapter gives a detailed account on the compilation and collection of the Quran by the Commander of the Faithful, Ali (a.s) following the demise of the Holy Prophet (s) during the early period of Abu Bakr's caliphate.
In the fourth chapter, the author elaborates on Imam Ali's understanding of the Quranic concepts and realities and his exclusive knowledge about the revelation of the verses on various occasions.
The fifth chapter provides a brief account of the causes of the revelation of verses about the merits and excellences of Imam Ali (a.s.) and his progeny. The details come in the body of the book.
The sixth and the last chapter explains the hadith which says that the tangible manifestation of the verses "O' you who believe…", which has been repeated in 90 verses of the Quran, is Ali (a.s.). He narrates the said hadith from different sources in a way such that the hadith s considered as "mutawatir".
Then the author enters the main body of the book beginning to discuss, step by step, the verses from chapter al-Fatihah to chapter al-Nas, those verses which have been revealed about the household of the Holy Prophet (s). Thereupon, he discusses the causes of the revelation and the verses that have been related through Shia and Sunni ways.
He shows no narrow-mindedness and prejudice in narrating the traditions from Sunni and Shia sources. Therefore, he refers, as far as he can, to all sources and references accessible to him.
Since the writer of Shawahid al-Tanzi is a Sunni narrator and lived in Sunni seminaries, worked there and Sunni sects prevailed in his place of residence, he has benefited profusely from Sunni narrative sources, books and narrators. It is for the same reason that this book is unique and distinct among other books in this respect.
C) Used References
The author has used all sources and references (chains) that have been at his disposal to compile this book. When we go through the traditions gathered in this book, we find the book to be comprehensive. Some of the people in the chains of the traditions mentioned in this book are the following:
1. The Holy Prophet (s), 2. Imam Ali (a.s), 3. Imam Hasan (a.s), 4. Imam Hussein (a.s), 5. Imam Baqir (a.s), 6. Imam Sadiq (a.s), 7 Abdullah bin Abbas from whom different people have reported including the following: 7 – 1. Mujahid, 7 – 2. Sa'eid bin Jubair, 7 – 3. Dhahhak, 7- 4. Abu Salih, 7 – 5. 'Akramah, 8. Jabir bin Abdullah Ansari, 9. Abdullah bin Mas'ud, 10. Salman Farsi, 11. Abu Abdullah al-Rahman Salami, 12. Sufyan Thawri, 13. Huzaifah, 14. Muqatil bin Sulayman, 15. Salim bin Qais, 16. Abu Sa'eid Khidri.
 - Extracted from Jami al-Tafasir Software produced by Islamic Sciences Computer Research (Noor) in Qom.