Musannaf Ibn Abi Shayba 3:4760

Abu Khalid al-Ahmar narrated to us, from Muhammad ibn ‘Ajlan, from Zayd ibn Aslam, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Imam was only appointed to be followed. So when he says 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar'. When he recites, then listen silently. When he says, 'Not of those who have evoked [Your] anger or of those who are astray,' then say 'Amin.' When he bows, then bow. When he says, 'Allah hears those who praise Him,' then say, 'O Allah, our Lord, to You be the praise.' When he prostrates, then prostrate. And when he prays sitting, then pray sitting."