Sahih Bukhari

Narrated Abu Huraira:Allah's Messenger (ﷺ) 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