Sahih al-Bukhari 2875

It is related that Sahl ibn Hathma al-Ansari said, "'Abdullah ibn Sahl and Muhayyisa ibn Mas'ud ibn Zayd went to Khaybar at a time when there was a truce. They parted and then Muhayyisa came upon 'Abdullah ibn Sahl, murdered and covered in blood. He buried him and then went to Madina. Then 'Abdu'r-Rahman ibn Sahl, and Muhayyisa and Huwayyisa, the sons of Mas'ud, went to the Prophet, may Allah bless him and grant him peace, and 'Abdu'r-Rahman began to speak. He said, 'The eldest, the eldest.' for he was the youngest of the people there. So he was silent, and the two of them spoke. He said, 'Do you swear (to this) and demand your right from the murderer - or your companion?' They asked, 'How can we swear when we did not witness it or see it?' He said, 'Then the Jews can clear themselves by fifty men taking an oath.' They said, 'How can you accept the oaths of a people of unbelievers!' So the Prophet, may Allah bless him and grant him peace, himself paid the blood money.'" XIII: The excellence of fulfilling the terms of a treaty