It was narrated from 'Amr bin Shu'aib, from his father, from his grandfather, that :
Huwayyisah and Muhayyisah , the sons of Mas'ud, and 'Abdullah and 'Abdur-Rahman the sons of Sahl, went out to search for food in Khaibar. 'Abdullah was attacked and killed, and mention of that was made to the Messenger of Allah (ﷺ). He said: "Will you swear an oath and establish your right to blood money?" They said: "O Messenger of Allah (ﷺ), how can we swear an oath when we did not witness anything?" He said: "Do you want the Jews to swear that they are innocent?" They said: "O Messenger of Allah (ﷺ), then they will kill us too." So the Messenger of Allah (ﷺ) paid the blood money himself.
English reference: Vol. 3, Book 21, Hadith 2678
Arabic reference: Book 21, Hadith 2780