Narrated 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 ﷺ, 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 Messenger of Allah ﷺ one night and came upon a man who made supplication with persistence. The Prophet ﷺ waited to hear him. The Prophet ﷺ 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 ﷺ 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.

Abu Dawud said: Al-Muqra'i is a clan of Himyar.

حَدَّثَنَا الْوَلِيدُ بْنُ عُتْبَةَ الدِّمَشْقِيُّ، وَمَحْمُودُ بْنُ خَالِدٍ، قَالاَ حَدَّثَنَا الْفِرْيَابِيُّ، عَنْ صُبَيْحِ بْنِ مُحْرِزٍ الْحِمْصِيِّ، حَدَّثَنِي أَبُو مُصْبِحٍ الْمَقْرَائِيُّ، قَالَ كُنَّا نَجْلِسُ إِلَى أَبِي زُهَيْرٍ النُّمَيْرِيِّ - وَكَانَ مِنَ الصَّحَابَةِ - فَيَتَحَدَّثُ أَحْسَنَ الْحَدِيثِ فَإِذَا دَعَا الرَّجُلُ مِنَّا بِدُعَاءٍ قَالَ اخْتِمْهُ بِآمِينَ فَإِنَّ آمِينَ مِثْلُ الطَّابَعِ عَلَى الصَّحِيفَةِ . قَالَ أَبُو زُهَيْرٍ أُخْبِرُكُمْ عَنْ ذَلِكَ خَرَجْنَا مَعَ رَسُولِ اللَّهِ ﷺ ذَاتَ لَيْلَةٍ فَأَتَيْنَا عَلَى رَجُلٍ قَدْ أَلَحَّ فِي الْمَسْأَلَةِ فَوَقَفَ النَّبِيُّ ﷺ يَسْتَمِعُ مِنْهُ فَقَالَ النَّبِيُّ ﷺ " أَوْجَبَ إِنْ خَتَمَ " . فَقَالَ رَجُلٌ مِنَ الْقَوْمِ بِأَىِّ شَىْءٍ يَخْتِمُ قَالَ " بِآمِينَ فَإِنَّهُ إِنْ خَتَمَ بِآمِينَ فَقَدْ أَوْجَبَ " . فَانْصَرَفَ الرَّجُلُ الَّذِي سَأَلَ النَّبِيَّ ﷺ فَأَتَى الرَّجُلَ فَقَالَ اخْتِمْ يَا فُلاَنُ بِآمِينَ وَأَبْشِرْ . وَهَذَا لَفْظُ مَحْمُودٍ . قَالَ أَبُو دَاوُدَ الْمَقْرَاءُ قَبِيلٌ مِنْ حِمْيَرَ .