This is a reference to the similarity between "seven years of servitude" in the report in Kulaynī, Kāfī, 6:196–97 and in Deuteronomy 15:12–15. Hossein Modarressi cites this source in the context of his selection of legal responses by Jaʿfar al-Ṣādiq on the maximum number of years of servitude by a believer, drawing attention to the similarity between the case in Jewish and 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).