Abu Dawud Book 34, Hadith Number 4509.

Narated By Rafi’ ibn Khadij : A man of the Ansar was killed at Khaybar and his relatives went to the Prophet (PBUH) and mentioned that to him. He asked: Have you two witnesses who can testify to the murderer of your friend? They replied: Apostle of Allah! there was not a single Muslim present, but only Jews who sometimes have the audacity to do even greater crimes than this. He said: Then choose fifty of them and demand that they take an oath; but they refused and the Prophet (PBUH) paid the blood-wit himself.