Musnad Ahmad 30:126

Sufyan ibn 'Uyaynah narrated to us, he said: We memorized it from al-Zuhri, from Sa'id, from Abu Hurayrah, attributing it to the Prophet, peace and blessings of Allah be upon him: "When the reciter says 'Amin', then say 'Amin', for the angels say 'Amin'. Whoever’s 'Amin' coincides with the 'Amin' of the angels, his past sins will be forgiven."