This is a reference to the recognized status of ʿAlī as a major authority on the Prophet's Sunna. Hossein Modarressi cites this reference to argue that ʿAlī was "the father" of the legal legacy of the Family of the Prophet.
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).