מדרש אויף בראשית 24:38

[4] Another interpretation: These are the generations of Isaac. But haven't we already heard "These are the generations of Ishmael" (Genesis 25:12)? Here we have "These are the generations of Isaac, son of Abraham". One might think that they are similar, God forbid, but "the Lord knows the way of the righteous" (Psalm 1:6). This can be compared to a donkey's foal that fell into a rose oil, and although its smell became pleasant from the rose oil, in the end it returns to its previous state. So too with the donkey's foal, which is the Egyptian handmaid, as it is written "Their flesh is the flesh of donkeys" (Ezekiel 23:20), she cleaved to Abraham and bore Ishmael, and it is written "These are the generations of Ishmael, son of Abraham." But in the end, she returned to her previous state, as it is written "And his mother took for him a wife from the land of Egypt" (Genesis 21:21), who was the Egyptian handmaid. However, look at what Abraham commanded for him, that his servant Eliezer should not take a wife for him from the daughters of Canaan, but rather from Abraham's family (Genesis 24:38). Therefore, "These are the generations of Isaac" and so on, but "Ishmael, these are the generations of Ishmael, whom Hagar the Egyptian, Sarah's maidservant, bore to Abraham" (Genesis 25:12).