Sunan an-Nasa'i, hadith 1063

أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ سُمَىٍّ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا قَالَ الإِمَامُ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ فَإِنَّ مَنْ وَافَقَ قَوْلُهُ قَوْلَ الْمَلاَئِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ ‏"‏ ‏.‏

It 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