سنن ابن ماجه

It was narrated that Abu Hurairah said: “The Messenger of Allah (ﷺ) said: ‘The Imam has been appointed to be followed, so when he says Allahu Akbar, then say Allahu Akbar, when he recites, then listen attentively; when he says: Not (the way) of those who earned Your anger, nor of those who went astray,[1:7] then say Amin; when he bows then bow; when he says Sami’ Allahu liman hamidah (Allah hears those who praise Him), then say Allahumma Rabbana wa lakal-hamd (O Allah, our Lord, to You is the praise);” when he prostrates then prostrate; and if he prays sitting down then all of you pray sitting down.’”