Sahih Bukhari Volume 5, Book 59, Hadith Number 513.

Narrated By ‘Abdul ‘Aziz bin Suhaib : Anas bin Malik said, “The Prophet took Safiya as a captive. He manumitted her and married her.” Thabit asked Anas, “What did he give her as Mahr (i.e. marriage gift)?” Anas replied. “Her Mahr was herself, for he manumitted her.”