Genesis 26:35

Which were a grief of mind unto Isaac and to Rebekah.

Esau took two wives from among the idolatrous Canaanites, specifically Hittites who worshiped false gods and idols. In marrying outside the chosen people he disobeyed God's command and broke a covenant condition that forbade intermarriage with idolaters and the heathen. This was the same principle Abraham had shown by his anxious care to secure for Isaac a wife of his own nation and faith. Their idolatry became a deep grief and sorrow of mind to both Isaac and to Rebekah.

This happened when Isaac was more than one hundred years old, with the infirmities of age upon him and his sight grown dim. Despite that sorrow Esau still remained Isaac's favorite son, loved more than Jacob. When Isaac thought he was about to die he asked Esau to prepare meat for him so that he could give him the blessing.