The Messenger of Allah ﷺ said, "When one of you says 'Ameen' while the angels in heaven say it as well, and the two coincide with one another, his past sins are forgiven."
وقال رسول الله ﷺ إذا قال أحدكم آمين والملائكة في السماء فيوافق إحداهما الأخرى غفر له ما تقدم من ذنبه
This is also sahih. It is in Sahifah Mufradah, number 10. Muslim also narrated it [2, 18 S/1-121 Bulaq] from Muhammad bin Rafi from Abdul-Razzaq, though he did not cite its wording, referring instead to the preceding one. Bukhari also narrated it 2:220, 1:223 Fath, from the copy of Araj, and Muslim likewise narrated it from that copy 1:120-121 Bulaq. Bukhari also narrated it by another route 2:221, 8:121 Fath, and Muslim narrated it by a third route 1:120. Something similar in meaning has already passed in another hadith from Abu Hurairah, 7187, 7243, 7647.As for his saying, "so one of the two matches the other," this is what is established in the original copies of Musnad and Jami al-Masanid. In Sahifah Mufradah it reads "fa-wafaqa," a past-tense verb, and I fear this may be an error in reading that manuscript.