Sahih Muslim Book - 4, Hadith - 920

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا يَعْقُوبُ يَعْنِي ابْنَ عَبْدِ الرَّحْمَنِ عَنْ سُهَيْلٍ عَنْ أَبِيهِ عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِذَا قَالَ الْقَارِئُ { غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ } فَقَالَ مَنْ خَلْفَهُ آمِينَ فَوَافَقَ قَوْلُهُ قَوْلَ أَهْلِ السَّمَاءِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ

It was narrated from Abu Hurairah that the Messenger of Allah (s.a.w) said: "When the reciter says: '(Not (the way) of those who earned Your Anger, nor of those who went astray' and those behind him say Amin; if a person's saying it coincides with that of the inhabitants of the heavens, his previous sins will be forgiven."