And he said: When the imam says "Sami' Allahu liman hamidah" and the follower says "Allahumma Rabbana wa lakal hamd," and his statement coincides with the "Allahumma Rabbana lakal hamd" of those in the heavens, then all his past sins will be forgiven.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9401