It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him); he said: Abu Talhah (may Allah be pleased with him) married Umm Sulaym (may Allah be pleased with her), and the dowry between them was accepting Islam. Umm Sulaym (may Allah be pleased with her) had become a Muslim before Abu Talhah; he proposed marriage to her, whereupon she said: "I have become a Muslim; if you become a Muslim, I will marry you." So he became a Muslim, and the dowry between them was accepting Islam itself.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21736