Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) said: '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; if he prays standing, then pray standing, and if he prays sitting down then pray sitting down." Daif