The Prophet ﷺ said, "When the imam recites 'not of those who have incurred Your wrath, nor of those who have gone astray' [Fatihah 1:7], say 'Ameen,' for the angels say 'Ameen' and the imam says 'Ameen.' Whoever's 'Ameen' coincides with that of the angels will have his past sins forgiven."
حدثنا عبد الأعلى عن معمر عن الزهري عن سعيد بن المسيب وعن أبي سلمة بن عبد الرحمن أنهما حدثاه
أن النبي ﷺ قال إذا قال الإمام {غير المغضوب عليهم ولا الضالين} فقولوا آمين فإن الملائكة يقولون آمين وإن الإمام يقول آمين فمن وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه
Its isnad is sahih. Nasai related it, 1:147, through the route of Yazid bin Zuray from Mamar from Zuhri from Saeed bin Musayab from Abu Hurairah, without mentioning Abu Salama. Malik related it in Muwatta, page 87, from Zuhri from Ibn Musayab and Abu Salama from Abu Hurairah, in abbreviated form, with the wording, "When the imam says amin, say amin," and so on. Bukhari and Muslim and others related it through the route of Malik. See Muntaqa, 903, 904. Al-Hafiz pointed to this narration of Mamar in Fath al-Bari, 2:218–219.