It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) freed Sayyidah Safiyyah (may Allah be pleased with her) and made her freedom her dowry (mahr).
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20551