This is a reference to an alleged meeting that took place between Abū Ḥanīfa (the eponym of the Ḥanafī school) and Jaʿfar al-Ṣādiq in the presence of the Abbasid caliph Manṣūr (r. 136-58) where Abū Ḥanīfa acknowledges Jaʿfar as being the most knowledgable person in religious law. Hossein Modarressi employs this reference to show how other Muslim scholars recognized Jaʿfar's mastery of Islamic law.
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).