Musannaf Ibn Abi Shayba 18:1092

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Yunus, from al-Hasan, regarding the one killed by treachery (ghilah). He said: Fifty oaths shall be sworn by those who are accused, [saying]: "We did not kill him, nor do we know the killer." If they swear, they are acquitted. If they refuse [to swear], then fifty [oaths] shall be sworn by the claimants, [saying]: "Our blood is upon you," and then they shall be paid the blood money.