This is a reference to an example of belief in the legality of wiping over one's boots in ritual ablution as part of Sunnī creed. Hossein Modarressi cites this reference to show how belief in the legality of wiping over one's boots had become a sectarian issue.
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).