Musannaf Ibn Abi Shaybah

Ibrahim says that I used to stand behind Hazrat Aswad until the muezzin would give the call to prayer and come down.

Ibrahim says that one should stand behind the imam until the bowing (ruku‘); if someone else comes, that is fine, otherwise, one should stand to the right side of the imam.

Ibrahim says that I used to stand behind Hazrat Alqamah until someone would enter the mosque or the muezzin would come down.