Musnad Ahmad 30:2796

I read to Abd al-Rahman: Malik, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "When one of you says: 'Amin,' the angels in the heaven say: 'Amin.' If one of them coincides with the other, his past sins are forgiven."