that the Messenger of Allah (ﷺ) set Safiyyah free, and made her ransom her dowry, and he married her.
Sunan Ibn Majah Book 9, Hadith 114
← Previous Hadith “Safiyyah was given to Dihyah Al-Kalbi (as his share of the war booty), then she was given to the Messenger of Allah after that. He married her, and made her ransom (i.e., freedom from slavery) her dowry.” (Sahih)Hammad said: “Abdul-'Aziz said to Thabit: 'O Abu Muhammad! Did you ask Anas what her bridal-money was?' He said: 'Her bridal-money was her freedom.' ”

Sunan Ibn Majah Book 9, Hadith 113 Next Hadith → that the Messenger of Allah said : "If a slave gets married without his master's permission, he is a fornicator."

Sunan Ibn Majah Book 9, Hadith 115