Musannaf Ibn Abi Shayba 18:1095

Abu Bakr narrated to us, he said: Shababa bin Sawwar narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, that the Prophet (may Allah bless him and grant him peace) decreed regarding the *qasamah* (oath-taking in cases of homicide) that the oath is upon the defendants.