Sahih Muslim Book 4, Hadith Number 809.

Chapter : Blessing on the Prophet (may peace be upon him) after Tashahhud.

Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: When the Imam says: “Allah listens to him who praises Him.” you should say: “O Allah, our Lord for Thee is the praise.” for if what anyone says synchronises with what the angels say, his past sins will be forgiven.