Saying “Āmīn” aloud by the Imam and the congregation:
عَنْ أَبِي هُرَيْرَةَ رضي الله عنه:
«إِذَا قَالَ الْإِمَامُ غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ، فَقُولُوا آمِينَ، فَإِنَّهُ مَنْ وَافَقَ قَوْلُهُ قَوْلَ الْمَلَائِكَةِ، غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ».
From Abu Hurayrah (may Allah be pleased with him):
The Messenger of Allah ﷺ said: “When the Imam says: ‘not of those who incurred Your anger, nor of those who went astray’, then you should say ‘Āmīn’. For whoever’s saying of ‘Āmīn’ coincides with that of the angels, his past sins will be forgiven.”
(Sahih al-Bukhari, Hadith 782)
حَدَّثَنَا مُحَمَّدُ بْنُ الصَّبَّاحِ، ‏‏‏‏‏‏وَعَمَّارُ بْنُ خَالِدٍ الْوَاسِطِيُّ، ‏‏‏‏‏‏قَالَا:‏‏‏‏ حَدَّثَنَا أَبُو بَكْرِ بْنُ عَيَّاشٍ، ‏‏‏‏‏‏عَنْ أَبِي إِسْحَاق، ‏‏‏‏‏‏عَنْعَبْدِ الْجَبَّارِ بْنِ وَائِلٍ، ‏‏‏‏‏‏عَنْ أَبِيهِ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ صَلَّيْتُ مَعَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏فَلَمَّا قَالَ:‏‏‏‏ وَلا الضَّالِّينَ قَالَ:‏‏‏‏ آمِينَ، ‏‏‏‏‏‏فَسَمِعْنَاهَا.
Narrated by Muhammad ibn al-Sabbah and ‘Ammar ibn Khalid al-Wasiti, who said: Abu Bakr ibn ‘Ayyash narrated to us, from Abu Ishaq, from ‘Abd al-Jabbar ibn Wa’il, from his father, who said:
“I prayed with the Prophet ﷺ, and when he said ‘wa lā al-dhāllīn’ (nor of those who went astray), he said ‘Āmīn’, and we heard it.”
(Sunan Ibn Majah, Hadith 855)