The Messenger of Allah ﷺ said, "When one of you says 'Ameen' and the angels in heaven say 'Ameen,' and the two coincide, his previous sins are forgiven."
قال قرأت على عبد الرحمن: مالك عن أبي الزناد عن الأعرج
أن رسول الله ﷺ قال إذا قال أحدكم آمين قالت الملائكة في السماء آمين فوافقت إحداهما الأخرى غفر له ما تقدم من ذنبه
Its isnad is sahih. "My father narrated to me" has dropped out of T, and it is certain that the one who recited to Abdul-Rahman bin Mahdi was Imam Ahmad, as it appears in the rest of the copies, and it will come in its correct form at 9890.