Muwatta Imam Malik Book of Prayer, Hadith 47

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)."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، وَأَبِي، سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ أَنَّهُمَا أَخْبَرَاهُ عَنْ أَبِي هُرَيْرَةَ، 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 .