It was narrated from Anas that the Messenger of Allah manumitted Safiyyah and made that her dowry.
Sunan an-Nasa'i Book 26, Hadith 147
← Previous Hadith "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."

Sunan an-Nasa'i Book 26, Hadith 146 Next Hadith → It was narrated from Anas that the Messenger of Allah manumitted Safiyyah and made her freedom her dowry.

Sunan an-Nasa'i Book 26, Hadith 148