This is the line of Ishmael, Abraham’s son, whom Hagar the Egyptian, Sarah’s slave, bore to Abraham.
Genesis 25:12 (The Contemporary Torah, JPS 2006)This is the story of Isaac, son of Abraham. Abraham begot Isaac.
Genesis 25:19 (The Contemporary Torah, JPS 2006)
Genesis 25:12 reinforced Ishmael’s mother’s identity. It is Hagar.
But Genesis 25:19 repeats Abraham’s role, rather than identity Sarah.
Why?
According to a teaching in the Midrash, it’s because nobody disputed Ishmael’s parents: it was Abraham and Hagar. But there were questions about whether or not Isaac was really the son of… Abimelech.
We, the readers, know the answer, but the people in the story needed to be convinced.
The sages say that a clue is in the word הוֹלִיד (holid) which we translate “sired” or “begot.” The numerical value is identical to דּוֹמֶה (domeh), which means “look alike.”
So the teaching is that when people saw Isaac, they all noted that he looked exactly like Abraham. There was no question: Abraham begot Isaac.