Sahih Bukhari Volume 4, Book 54, Hadith Number 451.

Narated By 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.”