It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When the Imam says Ameen, you should also say Ameen, because the angels also say Ameen. Whoever's Ameen coincides with the Ameen of the angels, his previous sins are forgiven."
Hadith Referenceالسنن المأثورة / باب ما جاء في القراءة في الركوع والسجود / 162
Hadith Takhrijصحیح بخاری، الدعوات، باب التأمين، رقم: 6402