Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates this statement of the Prophet (peace and blessings be upon him): "When the Imam says «سَمِعَ اللهُ لِمَنْ حَمِدَهُ», then you people recite «اللَّهُمَّ رَبَّنَا وَلَكَ الْحَمْدُ». Whoever's recitation coincides with that of the angels, his past sins will be forgiven."