Usul al-Kafi is a significant collection of hadith in Shia Islam, compiled by the scholar Abu Ja'far Muhammad ibn Ya'qub al-Kulayni. It serves as a primary source of Islamic teachings, covering various aspects of faith, ethics, and law. The work is divided into three sections: Usul al-Kafi, which deals with theological principles; Furu' al-Kafi, focusing on practical laws; and Ruh al-Kafi, which contains narrations on ethics and supplication. The compilation is highly regarded for its authenticity and comprehensiveness, making it an essential reference for Shia Muslims. Scholars and students of Islamic studies often consult Usul al-Kafi to gain insights into the teachings of the Ahl al-Bayt, the family of the Prophet Muhammad.