Narrated Anas: The Prophet offered the Fajr Prayer near Khaibar when it was still dark and then said, "Allahu-Akbar! Khaibar is destroyed, for whenever we approach a (hostile) nation (to fight), then evil will be the morning for those who have been warned." Then the inhabitants of Khaibar came out running on the roads. The Prophet had their warriors killed, their offspring and woman taken as captives. Safiya was amongst the captives, She first came in the share of Dahya Alkali but later on she belonged to the Prophet. The Prophet made her manumission as her 'Mahr'.

Sahih Bukhari 5:59:512 - Sunni Hadith