The Messenger of Allah ﷺ said, "When the imam says, "not of those who have incurred Your wrath, nor of those who have gone astray" [Fatihah 1:7], say, 'Ameen,' for whoever's utterance coincides with that of the angels will have his past sins forgiven."
أن رسول الله ﷺ قال: إذا قال الإمام: {غير المغضوب عليهم ولا الضالين} فقولوا: آمين، فإنه من وافق قوله قول الملائكة غفر له ما تقدم من ذنبه.
Malik narrated it in the Book of Prayer (45) from Sumayy, freedman of Abu Bakr, from Abu Salih Samman, from Abu Hurairah, and he related the hadith. Via Malik's route, Bukhari narrated it in the Book of Adhan (782) and in the Book of Tafsir (4475). Bukhari said, "Muhammad bin Amr, from Abu Salama, from Abu Hurairah, from the Prophet ﷺ, and Nuaym Mujmir, from Abu Hurairah, corroborated him."I say, the hadith of Abu Salama was narrated by Malik, and via his route by Bukhari in the Book of Adhan (780) and Muslim in the Book of Prayer from Malik (410), from Ibn Shihab, from Saeed bin Musayab and Abu Salama bin Abdul-Rahman, that both of them informed him from Abu Hurairah that the Messenger of Allah ﷺ said, "When the imam says ameen, say ameen, for whoever's ameen coincides with the ameen of the angels will have his previous sins forgiven." Ibn Shihab said, and the Messenger of Allah ﷺ used to say, "Ameen."Malik also narrated it from Abu Zinad, from Araj, from Abu Hurairah, that the Messenger of Allah ﷺ said, "When one of you says ameen, and the angels in the heavens say ameen, and the two coincide, his previous sins will be forgiven." Bukhari narrated it in the Book of Adhan (781) via the route of Malik, and Muslim narrated it from the hadith of Mughira, from Abu Zinad, in similar wording.As for the hadith of Nuaym Mujmir from Abu Hurairah, he said, Abu Hurairah led us in prayer, and when he reached "nor of those who have gone astray" [Fatihah 1:7], he said, "Ameen," and the people said, "Ameen." Then he said, "By the One in whose hand is my soul, I am the closest of you in resemblance to the Messenger of Allah ﷺ in prayer." Nasai narrated it (2/134).Ibn Khuzayma said, regarding the saying of the Prophet ﷺ, "When the imam says ameen, say ameen," it has become clear and established that the imam says ameen aloud, since it is known to anyone who understands knowledge that the Prophet ﷺ would not command the one following the imam to say "ameen" when the imam says ameen unless the follower knows that the imam has said it. Had the imam said "ameen" silently, without saying it aloud, the follower would not know whether his imam had said "ameen" or not. It is impossible to tell a man, "When so-and-so says such-and-such, say the same as what he said," while you cannot hear what he said. This is the very definition of impossibility. No scholar could imagine that the Prophet ﷺ would command the follower to say "ameen" when his imam says it, while the follower cannot hear his imam's ameen. (2/286).