Sahih Muslim 410
Sahih Muslim · Book 4: The Book of Prayers · Saying "Sami'a Allahu liman Hamidah," Rabbana wa lakal-hamd," and "Amin"
Abu Huraira reported:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا يَعْقُوبُ، - يَعْنِي ابْنَ عَبْدِ الرَّحْمَنِ - عَنْ سُهَيْلٍ ، عَنْ أَبِيهِ ، عَنْ أَبِي هُرَيْرَةَ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا قَالَ الْقَارِئُ غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ . فَقَالَ مَنْ خَلْفَهُ آمِينَ . فَوَافَقَ قَوْلُهُ قَوْلَ أَهْلِ السَّمَاءِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
The Messenger of Allah (ﷺ) said: When the reciter (Imam) utters:" Not of those on whom (is Thine) wrath and not the erring ones," and (the person) behind him utters Amin and his utterance synchronises with that of the dwellers of heavens, all his previous sins would be pardoned.