Musnad Ahmad 30:542

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) 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 the angels say: 'Amin,' and the Imam says: 'Amin.' So whoever’s 'Amin' coincides with the 'Amin' of the angels, his past sins will be forgiven."