Hazrat Abdullah said that when the Imam says "Sami' Allahu liman hamidah," the follower should say "Allahumma Rabbana lakal hamd."

Hazrat Aamir says that the follower should not say "Sami' Allahu liman hamidah" behind the Imam, rather he should say "Allahumma Rabbana lakal hamd."

It is narrated from Hazrat Abu Saeed that the Messenger of Allah, peace and blessings be upon him, said: When your Imam says "Sami' Allahu liman hamidah," you should say "Allahumma Rabbana lakal hamd."

Muhammad says that when the Imam says سَمِعَ اللَّہُ لِمَنْ حَمِدَہُ, his followers should say سَمِعَ اللَّہُ لِمَنْ حَمِدَہُ, اللَّہُمَّ رَبَّنَا لَکَ الْحَمْدُ.

A Madinan companion narrates that the Messenger of Allah, peace and blessings be upon him, heard the sound of my shoes while in prostration. When the prayer was finished, he said, "Whose shoe sound did I just hear?" I replied, "It was me." He said, "Then what did you do?" I said, "I found you in the state of prostration, so I also performed prostration with you." The Messenger of Allah, peace and blessings be upon him, said, "Do like this, and do not count that rak‘ah. Whoever finds me in the state of bowing, prostration, or standing, should join me in that state."

It is narrated similarly through another chain.

Hazrat Ibn Umar (may Allah be pleased with both of them) and Hazrat Zaid bin Thabit say that if a person finds the people in such a state that they have already raised their heads from bowing (ruku‘), then he should say Allahu Akbar and perform the prostration (sajdah), and he should not count this rak‘ah.

Ibrahim, regarding the person who finds the imam in the state of prostration, says that he should follow him and prostrate with him. He should not act in opposition to the imam. Also, due to the prostrations, he should not count that unit (rak‘ah); however, if he finds the imam in bowing (rukū‘), then the unit is counted.

Ibrahim says that whatever state you find the Imam in, do not oppose him.

Hazrat Qatadah says that if you find people in the state of prostration, then perform prostration with them, but do not count that rak‘ah.