Musannaf Ibn Abi Shayba 35:389

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Sa'id, that the *qasamah* (oath-taking procedure in homicide cases) existed in the Pre-Islamic period, and the Prophet (may Allah bless him and grant him peace) affirmed it regarding a man killed from the Ansar who was found in a well belonging to the Jews. He said: The Prophet (may Allah bless him and grant him peace) began with the Jews and charged them with the *qasamah* of fifty [oaths]. The Jews said: "We will not swear." The Prophet (may Allah bless him and grant him peace) then said to the Ansar: "Will you swear?" The Ansar said: "We will not swear." The Prophet (may Allah bless him and grant him peace) then imposed upon the Jews the payment of his blood money, because he was killed among them.