He—may Allah have mercy on him—was also asked:

A man found a congregation performing the noon prayer and wished to join them to make up the dawn prayer. When the imam rose for the third rak‘ah, he ceased following him and concluded his own prayer with the salutation of peace. Is this prayer valid? According to which school is it valid?

He answered:

This prayer is not valid according to the schools of Abū Hanīfah and Mālik, or according to one of the two reports from Ahmad. It is valid according to the school of al-Shāfi‘ī and the other report from Ahmad. And Allah knows best.

He was also asked:

A man found the congregational prayer underway and intended to follow the imam, thinking that he was Zayd, but it turned out that he was Amr. Does this affect the validity of his prayer? Likewise, what if the imam made a similar mistake about someone praying behind him?

He answered:

If his intention was to pray behind the imam of that congregation, whoever