Musannaf Ibn Abi Shayba 13:1480

Abu Bakr narrated to us, he said: Ma'n ibn 'Isa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, that he said regarding a group of people: If their testimonies differ, and they are equal in uprightness and number, then the oath is upon the one against whom the claim is made.