Sahih Muslim Book 4, Hadith Number 831.

Chapter : The reciting of Tasmi’ (Allah listens to him who praises Him) Tahmid (O our Lord, for Thee is the praise), and Timin (Amin).

Abu Huraira reported Allah’s Messenger (may peace be upon him) saying: The Imam is appointed to be followed. So recite takbir when he recites it, and bow down when he bows down and when he utters: “Allah listens to him who praises Him,” say “O Allah, our Lord for Thee be the praise.” And when he prays, standing, you should pray standing. And when he prays sitting, all of you should pray sitting.