Musannaf Ibn Abi Shayba 18:1119

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-Shaybani, from Hammad, from Ibrahim, who said: "If the Qasamah (oath-taking procedure in homicide cases) does not reach [the required number], they shall repeat [the oaths] until they have sworn fifty oaths."