Musnad Ahmad 30:2206

Muhammad ibn Ja'far narrated to us, he said: Muhammad ibn 'Amr narrated to us, from Abu Salamah, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Imam is only appointed to be followed. So when he says 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar'. When he bows, then bow. And when he says 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say 'Rabbana wa lakal-hamd' (Our Lord, and to You is the praise). And if he prays sitting, then pray sitting, all of you."