Musannaf Ibn Abi Shayba 18:1103

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from al-Shaybani, from al-Sha’bi, who said: A slain person was found in Yemen, in [the tribe of] Wada’ah. So, the matter was raised to Umar. He made them swear fifty oaths: "We did not kill [him], nor do we know the killer." Then, he paid his blood money.