Sahih Al Bukhari Hadith Number 796

Narrated Abu Huraira: Allah’s Messenger (peace be upon him) said, “When the Imam says, “Sami`a l-lahu liman hamidah,” 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.”