This is a reference to ʿAlī's refusal to adopt the practice of anyone beside the Prophet, including the caliphs who preceded him when he was offered the caliphate after the death of ʿUmar. Hossein Modarressi cites this reference to argue that ʿAlī understood the Sunna to be restricted to the Prophet's actions, decisions, and endorsements.
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).