Musannaf Ibn Abi Shayba 18:1082

Abu Muhammad Abdullah ibn Yunus narrated to us, he said: Abu Abd al-Rahman Baqi ibn Makhlad narrated to us, he said: Abu Bakr narrated to us, he said: Abd al-A’la ibn Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, from Sa’id ibn al-Musayyib: That the *qasamah* (oath-taking procedure for homicide) existed in the Pre-Islamic period (Jahiliyyah), and the Prophet, peace and blessings of Allah be upon him, affirmed it regarding a slain man from the Ansar who was found in the well of the Jews. He said: The Messenger of Allah, peace and blessings of Allah be upon him, began with the Jews and charged them with the *qasamah* of fifty [oaths]. The Jews said: "We will not swear." The Messenger of Allah, peace and blessings of Allah be upon him, said to the Ansar: "Will you swear?" The Ansar refused to swear. So, the Messenger of Allah, peace and blessings of Allah be upon him, imposed the blood money upon the Jews because he was killed in their midst.