Sayyiduna Abu Hurairah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: When the Imam says "ghayril-maghdoobi 'alayhim wa la-d-dallin," then you should also say Ameen, because whoever says Ameen along with the angels, all his previous sins are forgiven.
Hadith Referenceاللؤلؤ والمرجان / كتاب الصلاة / 231
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 10 كتاب الأذان: 113 باب جهر المأموم بالتأمين»