Narrated Abu Huraira:
Allah’s Apostle said, ”Whem the Imam says, ”Samiallahu Liman hamida,” you should say, ”Allahumma Rabbana Lakal Hamd,” and if the saying of anyone of you coincides with that of the angels, all his past sins will be forgiven.”
[Bukhari Vol 1 Book 12, Hadith 762]