Abu Dawud Book 34, Hadith Number 4511.

Chapter : Abandonment of retaliation in the case of qasamah taking oaths.

Narated By Some men : The Prophet (PBUH) said to the Jews and started with them: Fifty of you should take the oaths. But they refused (to take the oaths). He then said to the Ansar: Prove your claim. They said: Do we take the oaths without seeing, Apostle of Allah? The Apostle of Allah (PBUH) then imposed the blood-wit on the Jews because he (the slain) was found among them.