Musnad Ahmad 30:2554

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to teach us not to precede the Imam in bowing and prostrating. "And when he says 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar', and when he prostrates, then prostrate. And when he says: {Not of those who have evoked [Your] anger or of those who are astray} [Al-Fatihah: 7], then say: 'Amin'. For indeed, if it coincides with the speech of the angels, the one who is in the mosque will be forgiven. 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)."