Sayyiduna Abu Hurairah, may Allah be pleased with him, narrated that the Noble Prophet, peace and blessings be upon him, said: When the Imam says "Sami' Allahu liman hamidah," you should say "Allahumma Rabbana wa lakal hamd," for whoever's saying coincides with that of the angels, all his past sins will be forgiven.
Hadith Referenceاللؤلؤ والمرجان / كتاب الصلاة / 229
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 10 كتاب الأذان: 125 باب فضل اللهم ربنا ولك الحمد»
Sayyiduna Abu Hurairah, may Allah be pleased with him, narrated that the Messenger of Allah, peace and blessings be upon him, said: When any one of you says Ameen and the angels in the heavens also say Ameen at the same time, so that one’s Ameen coincides with the other’s Ameen, then all his previous sins are forgiven.
Hadith Referenceاللؤلؤ والمرجان / كتاب الصلاة / 230
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 10 كتاب الأذان: 112 باب فضل التأمين»
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»