Abu Hurairah (ra) Allah's Messenger (saw) said, "WHEN THE IMAM SAYS : GHAIRIL MAGHDUBI ALAIHIM WALAD DALLIN ( i.e. not the way of those who earned your Anger, nor the way of those who went astray ( Qur'an 1:7 ), THEN YOU MUST SAYS, AMEEN FOR IF ONE'S UTTERANCE OF AMEEN COINCIDES WITH THAT OF THE ANGELS, THEN HIS PAST SINS WILL BE FORGIVEN."
Sahih Al-Bukhari, Vol. 6 Hadith No. 2