Musnad Ahmad 30:2676

Yazid narrated to us, he said: Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the reciter says: {Not of those who have evoked [Your] anger or of those who are astray} [Al-Fatihah: 7], and those behind him say: 'Amin,' and that coincides with the saying of the people of the heaven: 'Amin,' his past sins will be forgiven."