Sunan an-Nasa'i 1063
سنن النسائي#1063SahihIt was narrated from Abu Hurairah that:The Messenger of Allah (ﷺ) said: "When the Imam says: 'Sami Allahu liman hamidah (Allah hears the one who praises Him),' then say: 'Rabbana wa lakal-hamd,' (Our Lord, and to You be the praise).' Whoever says that and it coincides with the angels saying it, his previous sins will be forgiven
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 12:35