Sahih Muslim Book 4, Hadith Number 826.

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: The Messenger of Allah (may peace be upon him) said: The Imam is appointed, so that he should be followed, so don’t be at variance with him. Recite takbir when he recites it; bow down when he bows down and when he says: “Allah listens to him who praises Him,” say: “O Allah, our Lord, to Thee be the Praise.” And when he (the Imam) prostrates, you should also prostrate, and when he says prayer sitting, you should all observe prayer sitting.