This is a reference to a report by Jaʿfar al-Ṣādiq that indicates the importance of performing each daily prayer in its respective best time. Hossein Modarressi cites this reference to show that, although al-Ṣāḍiq and his father al-Bāqir endorsed the practice of the Prophet that permitted combining prayers (outside of special circumstances), they also emphasized the importance of performing each prayer separately during its time. This is another example of differing legal practices between the Shīʿa and other Muslims.
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).