It was narrated that Anas said:
"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.' "
English reference: Vol. 3, Book 9, Hadith 1957
Arabic reference: Book 9, Hadith 2033