Mukhtasar Bukhari

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings of Allah be upon him) said: "When the Imam says 'Sami' Allahu liman hamidah', you should say 'Allahumma Rabbana lakal hamd.' Whoever's saying coincides with the saying of the angels (i.e., is in unison), all his past sins will be forgiven."