Musannaf Ibn Abi Shayba 18:1115

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: al-Mas’udi narrated to us, from al-Qasim, who said: Two men from the people of Kufa set out to Umar ibn al-Khattab and found that he had departed from the House (the Kaaba). They said: "O Commander of the Faithful, a cousin of ours has been killed, and we are equal in the blood (right to retaliation) regarding him." He remained silent toward them. Then he said: "Bring two just witnesses against the one who killed him, and we will grant you retaliation against him."