Sahih Muslim Book 4, Hadith Number 830.

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: Verily the Imam is a shield, say prayer sitting when he says prayer sitting. And when he says: “Allah listens to him who praises Him,” say: “O Allah, our Lord, to Thee be the praise.” and when the utterance of the people of the earth synchronises with that of the beings of heaven (angels), all the previous sins would be pardoned.