Allah's Apostle said, When the Imam says, Sami' a-l-lahu Liman ...

Hadith 1:762

Narrated 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."