It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) freed Sayyidah Safiyyah (may Allah be pleased with her) and made her freedom her dowry (mahr).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13366
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