This is a reference to the use of the epithet al-Ṣādiq ("the truthful") to refer to Abū ʿAbd Allāh Jaʿfar b. Muḥammad during his lifetime.
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).