Yahya related to me from Malik from Ibn Shihab that Sa'id ibn al- Musayyab and Abu Salama ibn Abd ar-Rahman told him from Abu Hurayra that the Messenger of Allah ﷺ said, "When the imam says 'Amin', say 'Amin', for the one whose 'Amin' coincides with the 'Amin' of the angels - his previous wrong actions are forgiven him." Ibn Shihab said, "The Messenger of Allah ﷺ used to say 'Aameen' (extending it)."
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا أَمَّنَ الإِمَامُ فَأَمِّنُوا فَإِنَّهُ مَنْ وَافَقَ تَأْمِينُهُ تَأْمِينَ الْمَلاَئِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ قَالَ ابْنُ شِهَابٍ وَكَانَ رَسُولُ اللَّهِ ﷺ يَقُولُ آمِينَ