(It is Sunnah for) the Imam to say «آمين» aloud (in the audible prayers), and Ata bin Abi Rabah said that «آمين» is a supplication. And Abdullah bin Zubair (may Allah be pleased with them both) and those who prayed behind him would say «آمين» so loudly that the mosque would resound. And Abu Huraira (may Allah be pleased with him) used to say to the Imam, "Do not deprive us of «آمين» ." And Nafi' said that Ibn Umar (may Allah be pleased with them both) never left «آمين» and used to encourage people to do it as well. I also heard a hadith from him regarding this.

Narrated Abu Huraira: The Prophet said, "Say Amin" when the Imam says it and if the Amin of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Apostle used to Say "Amin."