Sahih Bukhari | Volume 1 | Hadith Number 762

Narrated by Abu Huraira
Allah's Apostle said, "When the Imam says, "Sami' a-l-lahu Liman hamida," you should say, "Allahumma Rabbana laka-l-hamd." And if the saying of any one of you coincides with that of the angels, all his past sins will be forgiven."