Musannaf Ibn Abi Shaybah

Ibrahim says that if a person joins the congregation and the imam has already made a mistake before him, then he should perform the prostration of forgetfulness (sajda sahw) with the imam and then complete his prayer.

Hasan also says likewise.

Hasan says that if a person joins the congregation and the imam has already made a mistake before him, then he should perform the prostration of forgetfulness (sajda sahw) with the imam and then complete his prayer.

Hazrat Dahhak also says the same.

Hazrat Ibn Sirin says that first he should complete his prayer and then perform the prostration of forgetfulness (sajdah sahw). Hasan says that first he should perform the prostration of forgetfulness and then complete his prayer.

Ibrahim says that he should perform the prostration with the imam, and when the imam finishes, then stand up and complete the remainder of the prayer.