Musnad Ahmad Hadith 8502

It is narrated from Abu Huraira (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: The imam is appointed only to be followed, so when he says the takbeer, you also say the takbeer, and do not say the takbeer before he does. When he bows (in ruku), you also bow, and do not bow before he does. When he says "Sami' Allahu liman hamidah," you say "Rabbana wa lakal hamd." When he prostrates (in sujood), you also prostrate, and do not prostrate before he does. And when he prays sitting, then all of you should also pray sitting.