Anas bin Malik (may Allah be pleased with him) was saying that the Prophet (peace and blessings be upon him) took Safiyyah (may Allah be pleased with her) captive, then freed her and married her. Thabit al-Bunani (may Allah have mercy on him) says: I asked Anas (may Allah be pleased with him): What was her dower? He said: Her dower was herself; he freed her and then married her.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13742