“The Imam is appointed to be followed. When he says
Allahu Akbar, then say Allahu Akbar; when he bows, then bow; when he
says Sami’ Allahu liman hamidah, then say Rabbana wa lakal-hamd;
when
he prostrates then prostrate; and if he prays sitting down then
pray
sitting down.”
Sunan Ibn Majah Book 5, Hadith 436