This is a reference to the location of Jaʿfar al-Ṣādiq's house in Medina, which can be found in the oldest partially-surviving account of the city by Ibn Shabba (d. 262 AH). Al-Ṣādiq's house was located southeast of the Prophet's mosque next to the house of the prominent Companion Abū Ayyūb al-Anṣārī (d. 52 AH).
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).