(125) CHAPTER. The superiority of saying Allahumma Rabbana lakal hamd (O Allah, Our Lord! All the praises and thanks are for You).
Narrated Abu Huraira: Allah's Apostle said, When the Imam says, Sami`a l-lahu liman hamidah, 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.