Musnad Abu Ya'la al-Mawsili 155:34

On the authority of Abu Hurayrah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Imam recites 'Not of those who have evoked [Your] anger or of those who are astray,' then the Imam says 'Amin,' so say 'Amin.' For the angels say 'Amin' upon his supplication. Whoever’s 'Amin' coincides with the 'Amin' of the angels, his past sins will be forgiven."