Musannaf Ibn Abi Shayba 18:1088

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Mughira, from Ibrahim, who said: If a slain person is found in a neighborhood, fifty men shall be taken from it, among whom are those who are accused. If they are fewer than fifty, the oaths are returned to them, the first then the next.