Musannaf Ibn Abi Shayba 35:89

Abu Khalid narrated to us, from Muhammad ibn Ajlan, from Zayd ibn Aslam, from Abu Salih, from Abu Hurayrah, who said: The Prophet (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. And 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. And when he says: 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Allahumma Rabbana wa lakal-hamd' (O Allah, our Lord, and to You is the praise). And when he prostrates, then prostrate. And if he prays sitting, then pray sitting." And he mentioned that Abu Hanifah said: "An Imam should not lead while he is sitting."