Musannaf Ibn Abi Shayba 35:390

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, who said: 'Umar ibn 'Abd al-'Aziz summoned me and asked me about the *qasamah* (oath-taking in homicide cases). He said: "It has occurred to me to abolish it, for the Bedouin testifies, and the absent man comes and testifies." I said: "O Commander of the Faithful, you will not be able to abolish it; the Prophet (peace and blessings of Allah be upon him) judged by it, as did the Caliphs after him."