Musnad Ahmad 30:2794

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Sumayy, the freed slave of Abu Bakr, from Abu Salih al-Samman, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "When the Imam says: {Not of those who have evoked [Your] anger or of those who are astray} [Al-Fatihah: 7], then say: 'Amin.' For whoever’s saying coincides with the saying of the angels, his past sins will be forgiven."