Sahih Muslim Book 4, Hadith Number 828.

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) while teaching us (the principles of faith), said: Do not try to go ahead of the Imam, recite takbir when he recites it and when he says: “Nor of those who err,” you should say Amin, bow down when lie bows down, and when he says: “Allah listens to him who praises Him,” say: “O Allah, our Lord, to Thee be the praise”.