This is a reference to an early definition of Shīʿism by the Kūfan scholar Abān b. Taghlib (d. 141): "The Shīʿa are those who follow the opinion of ʿAlī when reports from the Prophet are contradictory, and the opinion of Jaʿfar b. Muḥammad [al-Ṣādiq] when reports from ʿAlī are contradictory."
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).