Anas bin Malik said, "The Prophet (ﷺ) 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."
Sahih al-Bukhari Book 64, Hadith 241