Sunan an-Nasa'i, Book 26, Hadith 146 (In-Book) ⓘ

It was narrated that Anas said: "Abu Talhah proposed marriage to Umm Sulaim and she said: 'By Allah, a man like you is not to be rejected, O Abu Talhah, but you are a disbeliever and I am a Muslim, and it is not permissible for me to marry you. If you become Muslim, that will be my dowry, and I will not ask you for anything else.' So he became Muslim and that was her dowry." (one of the narrators) Thabit said: "I have never heard of a woman whose dowry was more precious than Umm Sulaim (whose dowry was) Islam. And he consummated the marriage with her, and she bore him a child."

أَخْبَرَنَا مُحَمَّدُ بْنُ النَّضْرِ بْنِ مُسَاوِرٍ، قَالَ أَنْبَأَنَا جَعْفَرُ بْنُ سُلَيْمَانَ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ خَطَبَ أَبُو طَلْحَةَ أُمَّ سُلَيْمٍ فَقَالَتْ وَاللَّهِ مَا مِثْلُكَ يَا أَبَا طَلْحَةَ يُرَدُّ وَلَكِنَّكَ رَجُلٌ كَافِرٌ وَأَنَا امْرَأَةٌ مُسْلِمَةٌ وَلاَ يَحِلُّ لِي أَنْ أَتَزَوَّجَكَ فَإِنْ تُسْلِمْ فَذَاكَ مَهْرِي وَمَا أَسْأَلُكَ غَيْرَهُ ‏.‏ فَأَسْلَمَ فَكَانَ ذَلِكَ مَهْرَهَا - قَالَ ثَابِتٌ فَمَا سَمِعْتُ بِامْرَأَةٍ قَطُّ كَانَتْ أَكْرَمَ مَهْرًا مِنْ أُمِّ سُلَيْمٍ الإِسْلاَمَ - فَدَخَلَ بِهَا فَوَلَدَتْ لَهُ ‏.‏