This is a reference to ʿAlī's description of the Sunna of the Prophet. Hossein Modarressi argues that in the legal legacy of the Family of the Prophet, the Prophet's Sunna was next to the Qurʾān in importance.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).