Hadith #4025 (Sahih al-Bukhari)
Book: Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Narrator(s): Abdul 'Aziz bin Suhaib عبد العزيز بن صهيب البناني
Grade: Sahih

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.”

Source: https://parallelquran.com/hadith/sahih-al-bukhari-4025/