Sahih Bukhari
Beginning of Creation
Volume: 004 / Book Number: 054 / Hadith Number: 451
Narrated Abu Huraira:
Allah's Apostle said, "When the Imam, during the prayer, says, "Allah hears him who praises Him', say: 'O Allah! Our Lord! All the praises are for You/, for if the saying of anyone of you coincides with the saying of the angels, his past sins will be forgiven."