Musnad Ahmad 30:1382

'Affan narrated to us, Wuhayb narrated to us, Mus'ab ibn Muhammad narrated to us, from Abu Salih al-Samman, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The Imam is only to be followed. So when he says the takbir, say the takbir, and do not say the takbir until he says it. When he bows, then bow, and do not bow until he bows. When he says, 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say, 'Rabbana wa laka al-hamd' (Our Lord, and to You is the praise). When he prostrates, then prostrate, and do not prostrate until he prostrates. And if he prays sitting, then pray sitting, all of you."