It is narrated from Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) freed Safiyyah (may Allah be pleased with her) and then married her. I asked Thabit: What was her dowry (mahr)? He said: Her own self.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13367