(a) It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the imam says Amin, then you too say Amin, for whoever's Amin coincides with the Amin of the angels, his previous sins will be forgiven."
(b) Ibn Shihab said: The Messenger of Allah (peace and blessings be upon him) used to say Amin.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2433