Sahih Muslim Book 16, Hadith Number 4123.

Bushair b. Yasar reported that ‘Abdullah b. Sahl b. Zaid and Muhayyisa b. Mas’ud b. Zaid, both of them were Ansar belonging to the tribe of Banu Haritha, set out to Khaibar during the lifetime of Allah’s Messenger (may peace be upon him). There was peace during those days and (this place) was inhabited by the Jews. They parted company for their (respective) needs. ‘Abdullab b. Sahl was killed, and his dead body was found in a tank. His companion (Muhayyisa) buried him and came to Medina, and the brothers of the slain ‘Abd al-Rahman b. Sahl. and Muhayyisa and Huwayyisa told Allah’s Messenger (may peace be upon him) the case of ‘Abdullah and the place where he had been murdered. Bushair reported on the authority of one who had seen Allah’s Messenger (may peace be upon him) that he had said to them: You take fifty oaths and you are entitled to blood-wit of (one) slain among you (or your companion). They said: Messenger of Allah, we neither saw (with our own eyes this murder) nor were we present there. Thereupon (Allah’s Messenger is reported to have said): Then the Jews will exonerate themselves by taking fifty oaths. They said: Allah’s Messenger, how can we accept the oath of unbelieving people? Bushair said that Allah’s Messenger (may peace be upon him) paid the blood-wit himself.