This is a reference to Jaʿfar al-Ṣādiq's knowledge of the various types of dates and his overall preference for the dates of the Ḥijāz versus those of Iraq. Hossein Modarressi cites this reference in his discussion of some of the features of al-Ṣādiq's personal life.
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).