Musannaf Ibn Abi Shayba 18:1098

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, from Sa'id ibn al-Musayyib, that he used to hold the view that the *qasamah* (oath-taking procedure in homicide cases) is upon the defendants.