This is a reference to a report by Jaʿfar al-Ṣādiq that holds the original call to prayer (adhān) to have included a call to "the best of acts" (ḥayya ʿalā khayr al-ʿamal). Hossein Modarressi cites this report as an example of one of the differences between the legal legacy of the House of the Prophet and other legal schools with respect to cases concerning prayer.
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).