This is a reference to al-Shāfiʿī's report indicating that the Prophet advised his followers to compare the statements attributed to him to the Qurʾān and to accept those that conform to the Qurʾān and reject those that do not. Hossein Modarressi cites this reference to show that the understanding among many early Muslims was that the Qurʾān was the criterion for testing the validity of all other sources.
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).