Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Abu Huraira that the Messenger of Allah, peace and blessings be upon him, said: The imam is appointed so that you may follow him. When he says the takbeer, you should also say the takbeer, and when he recites, you should remain silent. When he says "ghayril maghdubi 'alayhim wa la-d-dallin," you should say "Ameen." When he bows (in ruku'), you should bow. When he says "Sami' Allahu liman hamidah," you should say "Allahumma Rabbana lakal hamd." When he prostrates, you should prostrate, and when he prays sitting, you should also pray sitting.