This is a reference to Ṣaḥīfat al-farāʾiḍ, a text on the designated shares of inheritance recorded by ʿAlī from the statements of the Prophet, which Jaʿfar al-Ṣādiq possessed and showed to his disciples. Hossein Modarressi cites this reference to argue that ʿAlī was "the father" of the legal legacy of the Family of the Prophet and that the Prophet's teachings were passed down through the family from one generation to the next.
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).