The Messenger of Allah ﷺ said, "When the imam says 'not of those who have earned Your anger, nor of those who have gone astray' [Fatihah 1:7], say 'Ameen,' for the angels say 'Ameen' and the imam says 'Ameen.' Whoever's saying of 'Ameen' coincides with that of the angels will have his past sins forgiven."
حدثنا عبد الرزاق حدثنا معمر عن الزهري عن ابن المسيب
أن رسول الله 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. It has already passed at 7187 from Abdul-Ala, from Maamar, from Zuhri, from Ibn Musayab and Abu Salama together, from Abu Hurairah. It has also passed at 7243 in abridged form from Sufyan bin Uyayna, from Zuhri, from Ibn Musayab alone, from Abu Hurairah.As for his saying, "the angels say," this is what is established in the two manuscripts K and M. In H it reads "they say," and this is a variant reading noted in the margin of K and M.