(125) باب فَضْلِ اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ
Narrated Abu Huraira:
Allah's Messenger (ﷺ) 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."
Reference: Sahih al-Bukhari 796
In-book reference: Book 10, Hadith 191
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 12, Hadith 762