Rashi on Genesis 25:19/רש"י על בראשית כ״ה:י״ט

1ואלה תולדות יצחק AND THESE ARE THE PROGENY OF ISAAC — viz, Jacob and Esau who are spoken of in this section.

2אברהם הוליד את יצחק ABRAHAM BEGAT ISAAC — Just because Scripture wrote, “Isaac, son of Abraham” it felt compelled to say “Abraham begat Isaac”, because the cynics of that time said, “Sarah became with child of Abimelech. See how many years she lived with Abraham without becoming with child”. What did the Holy One, blessed be He, do? He shaped Isaac’s facial features exactly similar to those of Abraham’s, so that everyone had to admit that Abraham begat Isaac. This is what is stated here: that Isaac was the son of Abraham, for there is evidence that Abraham begat Isaac (Midrash Tanchuma, Toldot 1).