Musannaf Ibn Abi Shayba 17:1252

Mahbub al-Qawariri narrated to us, from al-Sa'b ibn Hakim, from his father, from his grandfather, who said: I came to Umar ibn al-Khattab, and he began saying, "O son of my brother." Then he asked me, so I told him my lineage. He realized that my father had not reached Islam, so he began saying, "O my son."