Musnad Ahmad

It is narrated from Hazrat Abu Musa Ash'ari (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) taught us the prayer and its method and said: The imam is appointed only to be followed, so when he says the takbeer, you also say the takbeer, and when he says "ghayril maghdubi 'alayhim wa la-d-dallin," then say "Ameen," Allah will accept it. When he bows (for ruku'), you also bow; when he raises his head, you also raise your heads. When he says "Sami' Allahu liman hamidah," you say "Rabbana lakal hamd," Allah will surely listen to you. When he prostrates, you also prostrate; when he raises his head, you also raise your heads, because the imam will prostrate before you and will raise his head before you; this will suffice in its place.