Hadith 501960
Chapter 3: Prayer - كتاب الصلاة
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا قَالَ أَحَدُكُمْ آمِينَ وَقَالَتِ الْمَلاَئِكَةُ فِي السَّمَاءِ آمِينَ فَوَافَقَتْ إِحْدَاهُمَا الأُخْرَى غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When one of you says 'Amin' and the angels in the sky say 'Amin' so that one coincides with the other, his previous wrong actions are forgiven him."
Reference Muwatta Malik 501960
USC-MSA web (English) reference : Book 3, Hadith 49