Musnad Ahmad 30:2278

The Prophet (peace and blessings of Allah be upon him) said: "When the reciter says: 'Allah hears those who praise Him' (Sami' Allahu liman hamidah), and those behind him say: 'O Allah, our Lord, to You be the praise' (Allahumma Rabbana laka al-hamd), and his saying coincides with the saying of the people of the heaven: 'O Allah, our Lord, to You be the praise,' his past sins will be forgiven."