It is narrated from Hazrat Anas bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) freed Hazrat Safiyyah (may Allah be pleased with her), daughter of Huyayy, and made her freedom itself her dowry.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11957