Musannaf Ibn Abi Shayba 35:344

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Sa‘id, from Abu Hurayrah, who attributed it to the Prophet (ﷺ), he 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."