Rashi on Genesis 36:5/רש"י על בראשית ל״ו:ה׳

1ואהליבמה ילדה וגו׳ AND OHOLIBAMAH BARE etc. — This Korah was an illegitimate child for he was the son of Eliphaz (cf. vv. 15, 16, 18) who had taken his father’s wife — Oholibamah, the wife of Esau — for he is enumerated also amongst the chiefs of Eliphaz at the end of this section (Genesis Rabbah 82:12).