Al-ʾUṣūl al-arbaʿamiʾa (Arabic: الأصول الأربعمائة, the four hundred principles) are four hundred hadith collections which were collected by the transmitters of hadith directly from the Prophet (s) and the Imams (a) in the early centuries of Islam.
They were sources for later hadith references; and one criterion of authenticity for next generation of hadith scholars was the existence of at least one of these "Usul" in the reference of a hadith.
The content of these collections was hadiths from the Infallible Imams (a) in different fields of rulings, traditions, sermons, supplications and commentaries of the Qur'an. Collecting them was being carried out by the companions from the time of Imam 'Ali (a) until Imam al-Hasan al-Askari (a) and the number of those companions has sometimes been reported to have been exactly four hundred, most of whom were companions of Imam Ja'far al-Sadiq (a).
Apparently, the trustworthiness of such Usul is that the companions of Imams (a) would instantly write down hadiths from them upon hearing them which lowered the possibility of forgetting them or modification of their wording. This claim is approved by Sayyid b. Tawus's report in Muhaj al-da'awat saying that, "a group of the companions of Imam al-Kazim (a) would attend his presence having pen and paper and recorded anything he said or any ruling he gave."Read more...