Musannaf Ibn Abi Shayba 18:1090

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Ibn Abi Layla narrated to us, from al-Sha'bi, that a slain person was found in Yemen between two tribes. He said: So Umar said, "Look for the tribe closest to him, and make fifty men from among them swear by Allah that they did not kill him and do not know the killer, then the blood money shall be upon them."