It was narrated from 'Aishah:
that the Messenger of Allah (ﷺ) set Safiyyah free, and made her ransom her dowry, and he married her. (RU)
English reference (RU): Vol. 3, Book 9, Hadith 1958 (RU)
Arabic reference (RU): Book 9, Hadith 2034 (RU)