Abu Misbah al-Muqra'i said: We used to sit with Abu Zuhayr an-Numayri—and he was from the Companions—and he would narrate the finest discourse. Whenever a man among us offered a supplication, he would say, 'Conclude it with "Amin", for "Amin" is like the seal upon the scroll.'
Abu Zuhayr said, 'I shall inform you about that: We went out with the Messenger of Allah (ﷺ) one night, and we came upon a man who had persisted in asking. The Prophet (ﷺ) stopped and listened to him. The Prophet (ﷺ) said, "He has made it incumbent [upon Allah to answer or grant Paradise], if he concludes." A man from the people asked, "With what thing should he conclude?" He said, "With 'Amin', for indeed if he concludes with 'Amin', he has made it incumbent." Then the man who had asked the Prophet (ﷺ) departed and came to the man, saying, "Conclude, O so-and-so, with 'Amin' and receive glad tidings."
This is the wording of Mahmud.
Abu Dawud said: Al-Muqra'i is a clan from Himyar.