Musannaf Ibn Abi Shayba 20:64

Muhammad ibn Bishr narrated to us, he said: Ibn Abi Arubah narrated to us, from Qatadah, from Sulayman ibn Yasar, who said: The Qasamah (oath-taking procedure in homicide cases) is a right which the Prophet, may Allah bless him and grant him peace, decreed. While the Ansar were with the Messenger of Allah, may Allah bless him and grant him peace, a man from among them went out. Then they (the others) went out from the Messenger of Allah, may Allah bless him and grant him peace, and suddenly they found their companion wallowing in his blood. They returned to the Messenger of Allah, may Allah bless him and grant him peace, and said: "The Jews have killed our companion," and they named a man among them, but they had no evidence. The Messenger of Allah, may Allah bless him and grant him peace, said to them: "Two witnesses from other than you, so that I may hand him over to you in his entirety (i.e., to be executed)." They did not have evidence, so he said: "Then establish the Qasamah, and I will hand him over to you in his entirety." They said: "We dislike swearing to what we have not seen." The Messenger of Allah, may Allah bless him and grant him peace, intended to take the Qasamah from the Jews by fifty of them. The Ansar said: "O Messenger of Allah, the Jews do not care about swearing; if we accept this from them, they will finish us off one by one." So the Messenger of Allah, may Allah bless him and grant him peace, paid the blood money for him from his own [resources].