It wasnarrated that Abu Hurairah said:
"The Messenger of Allah (ﷺ)said: 'The Imam has been appointed to be followed, so when he saysAllahu Akbar, then say Allahu Akbar, when he recites, then listenattentively; when he says: Not (the way) of those who earned Youranger, nor of those who went astray,[1:7] then say Amin; when he bowsthen bow; when he says Sami' Allahu liman hamidah (Allah hearsthosewho praise Him), then say Allahumma Rabbana wa lakal-hamd (OAllah,our Lord, to You is the praise);" when he prostrates thenprostrate;and if he prays sitting down then all of you pray sittingdown.'"
English reference: Vol. 1, Book 5, Hadith 846
Arabic reference: Book 5, Hadith 895