Thiqat al-islam Muhammad ibn Ya'qub Kulayni (d. 329/940) is the first person in Shi'ism to have separated the Shi'ite hadiths from the books called Principles (usul) and to have arranged and organized them according the headings of jurisprudence and articles of faith. (Each one of the Shi'ite scholars of hadith had assembled sayings he had collected from the Imams in a book called Asl, or Principles .) The book of Kulayni known as Kafi is divided into three parts: Principles, Branches, and Miscellaneous Articles, and contains 16,199 hadiths. It is the most trustworthy and celebrated work of hadith known in the Shi'ite world.
Three other works which complement the Kafi are the book of the jurist Shaykh-i Saduq Muhammad ibn Babuyah Qumi(d. 381/991), and Kitab al-tahdhib and Kitab al-istibsar , both by Shaykh Muhammad Tusi (d. 460/1068).