Musnad Ahmad 30:2793

I read to Abd al-Rahman ibn Mahdi: Malik, from Ibn Shihab, from Sa'id ibn al-Musayyib and Abu Salamah ibn Abd al-Rahman, that they informed him, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, 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."