These four books were the following...

These four books were the following: Al-Kafi: It was compiled by Shaykh Abu Ja`far Muhammad ibn Ya`qub al-Kulayni al-Razi (died 329/940) which contains 16,099 musnad (documented) hadith narrated from the (the Household of the Prophet).[^31] Man la yahduruhu al-faqih: It was compiled by Shaykh Saduq Abu Ja`far Muhammad ibn `Ali ibn Babwayhi al-Qummi (died 381/991) who is known as "Shaykh-i Ajal" or "Saduq al-Ta'ifah".

This book contains 9,044 hadith.[^32] Al-Tahdhib: It was compiled by Abu Ja`far Muhammad ibn al-Hasan al-Tusi (died 460/1068) also known as "Shaykh al-Ta'ifah" (The chief of the sect). This book contains 13,590 hadith.[^33] Al-Istibsar: This book was also compiled by Shaykh Tusi, and contains 5,511 hadith.

The book is divided into four parts.[^34] It is necessary to mention here that the four hundred "Usul" were widely quoted and narrated by the Shi`ah muhaddithin (scholars of hadith) until a comprehensive compilation called al-Mahasin was done by Shaykh Ahmad ibn Muhammad ibn Khalid al-Barqi, who died in the second half of the third century of Hijra. His book contained a large number of ahadith arranged in numerous chapters.

The al-Mahasin set an example which opened a new era in the history of the science of Shi`ah hadith;[^35] because it was after him that others took up the task of collection, compilation and classification of ahadith, which were until then scattered in hundreds of Usul. This trend led to the emergence of the four authoritative compilations of hadith during the fourth and fifth centuries.

Since then, they have been considered the greatest sources of hadith for the Shi`ah and served as the primary sources for the later day writers. The Age of Exposition: After the compilation of the four great texts of hadith, the next stage was that of exposition. During this period, the attention of most of the scholars was devoted to writing of commentaries and exposition of these texts. A large number of commentaries were written on each of these texts.