Musannaf Ibn Abi Shayba 3:5581

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Sa‘id, from Abu Hurayrah, who said: "When the reciter says 'Amin,' then say 'Amin.' For whoever’s 'Amin' coincides with the 'Amin' of the angels, his past sins will be forgiven."