Musannaf Ibn Abi Shayba 18:1086

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: al-Mas’udi narrated to us, from al-Qasim ibn ‘Abd al-Rahman, who said: Two men from the people of Kufa set out to ‘Umar ibn al-Khattab. They found that he had departed from the House (the Ka’bah), intending to go to Mina. So they performed Tawaf around the House, then they caught up with him and related their story to him. They said: "O Commander of the Faithful, a cousin of ours was killed, and we are equal to him in blood (i.e., kin)." He remained silent regarding them and did not reply to them with anything, until they adjured him by Allah. Then he turned upon them, but they reminded him of Allah, so he refrained from them. Then ‘Umar said: "Woe to us if we are not reminded by Allah, and woe to us if we do not remember Allah. Bring two just witnesses regarding the one who killed him, and we will exact retribution from him for you. Otherwise, let those whom you accuse swear by Allah: 'We did not kill him, nor do we know the killer.' If they refuse, then fifty of you shall swear, and then the blood money shall be yours."