Authentic Supplication Number 80.

It is reported by way of ‘Abi Hurayrata  that the Prophet  said: “If the Imam says ‘Aameen then say ‘Aameen for one, whose saying of ‘Aameen coincides with the Angels’ saying of ‘Aameen, would have what preceded of his sins forgiven for him.

Reporters.
‘Al-`Adawee said it was sound, and that it was reported by Al-Bukhari, page 262 of volume 2 Fath Al-Baree; Muslim, page 307 of the Fou’ad Abdil Baqee print; Al-Bayhaqi, 55/2; Ahmad; Abu ‘Uwanah, page 143 of volume 2; and others.