Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1064 (DarusSalam) ⓘ

It was narrated from Abu Hurairah that: The Messenger of Allah 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."

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 ‏" ‏ ‏.‏