Messenger of Allah ﷺ said, "When the Imam says: 'Ghair-il-Maghdubi ʿalaihim Walad-Dallin (i.e. not the path of those who earn Your Anger, nor the path of those who went astray (1.7)), then you must say, 'Ameen', for if one's utterance of 'Ameen' coincides with that of the angels, then his past sins will be forgiven." (Using translation from Bukhārī 4475)
"إذا قال القارئُ: {غَيرِ الْمَغْضُوبِ عَلَيهِمْ وَلَا الضَّالِّينَ} فقال مَنْ خلفَه (آمين) فوافق قوْلهُ قوْلَ أهْلِ السَّماءِ غُفِر له ما تقدَّم من ذنبِه".