Arabic (Original)
وَقَالَ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا قَالَ الْقَارِئُ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقَالَ مَنْ خَلْفَهُ اللَّهُمَّ رَبَّنَا وَلَكَ الْحَمْدُ فَوَافَقَ قَوْلُهُ ذَلِكَ قَوْلَ أَهْلِ السَّمَاءِ اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ.
English Translation
And he (peace and blessings of Allah be upon him) said: "When the imam says 'Sami Allahu liman hamidah' and those behind him say 'Allahumma Rabbana wa lakal-hamd,' and their words coincide with the words of the people of the heavens - 'Allahumma Rabbana lakal-hamd' - his past sins are forgiven."