Narated By AbuZuhayr an-Numayri : AbuMisbah al-Muqra’i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (PBUH), and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book.
AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah (PBUH) one night and came upon a man who made supplication with persistence. The Prophet (PBUH) waited to hear him. The Prophet (PBUH) said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him).
Then the man who questioned the Prophet (PBUH) came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news. These are the words of Mahmud.