Narrator:Abu Zuhayr an-Numayrī
وَعَن أبي زُهَيْر النميري قَالَ: خَرَجْنَا مَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ذَاتِ يَوْمٍ فَأَتَيْنَا عَلَى رَجُلٍ قَدْ أَلَحَّ فِي الْمَسْأَلَةِ فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: أَوْجَبَ إِنْ خَتَمَ ". فَقَالَ: رَجُلٌ مِنَ الْقَوْمِ: بِأَيِّ شَيْءٍ يَخْتِمُ؟ قَالَ: «بآمين» . رَوَاهُ أَبُو دَاوُد
Abu Zuhair an-Numairi said:
We went out with God’s Messenger one night and came upon a man who made supplication with persistence. The Prophet said, “He will have done something which guarantees [paradise for him] if he puts a seal to it.” One of the people asked what he should use for a seal, and he replied, “Amen”. Abu Dawud transmitted it.
Cited in Mishkat al-Masabih (No. 846).