This is a reference to the high standing status of Jaʿfar al-Ṣādiq's father Muḥammad al-Bāqir (57-114 AH) among his followers especially in Iraq. Hossein Modarressi employs this reference to argue that al-Bāqir was considered the spiritual leader of the pro-ʿAlid communities of Iraq in the late first and early second centuries of Islam.
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).