Al-Kutub al-Arbaʿa or al-Uṣūl al-Arbaʿa (Arabic:الکتب الاربعة، الاصول الاربعة) (the Four Books or the Four Principles) are the four books of hadith regarded as reliable by the Shi'as. The Four Books are: al-Kafi, Man la yahduruh al-faqih, Tahdhib al-ahkam, and al-Istibsar. Al-Kafi was written by al-Kulayni and Man la yahdur was written by al-Shaykh al-Saduq. Tahdhib al-ahkam and al-Istibsar were written by al-Shaykh al-Tusi.
The term, al-Kutub al-Arba'a, was first coined and used by al-Shahid al-Thani in a permission for the transmission of hadiths he gave to someone. Then the term began to be commonly used in jurisprudential texts. Some Shiite scholars regard all hadiths in the Four Books to be reliable. However, most of them restrict its reliable hadiths to those that are mutawatir or have reliable chains of transmitters.