Sahih Ibn Khuzaymah

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the Imam says Ameen, then you should also say Ameen. So whoever's saying Ameen coincides with the angels' saying Ameen, his previous sins will be forgiven." Imam Abu Bakr (may Allah have mercy on him) says that in the blessed statement of the Prophet (peace and blessings be upon him), "When the Imam says Ameen, then you should also say Ameen," there is evidence and it is proven from this that the Imam will say Ameen aloud, because a person of understanding knows well that the Prophet (peace and blessings be upon him) only ordered the follower to say Ameen at the time when the Imam says Ameen, when he hears his Imam saying Ameen. And if the Imam says Ameen quietly and does not say it aloud, then the follower will not know whether his Imam has said Ameen or not, and it is impossible and inconceivable that someone should be told: When so-and-so says such-and-such, you should also say the same, even though you cannot hear his statement. This is absolutely impossible and inconceivable. Such an impossible thing cannot even occur to the mind of a knowledgeable person, that the Prophet (peace and blessings be upon him) would order the follower to say Ameen at the time when his Imam says Ameen, even though he does not hear his Imam's Ameen. Imam Abu Bakr (may Allah have mercy on him) says: Now listen to that explicit and clear hadith which is proof of the correctness of our statement, that after reciting « سورة الفاتحة », he will say Ameen aloud.