Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 809
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.