TT #1 · Parashas Toldos · Passuk 25:19
This is the story of Yitzchak, son of Avraham. Avraham begot Yitzchak.
Torah Temimah points to Bava Metzia 87a. E-Daf · Sefaria — Bava Metzia 87a
When Sarah gave birth to Yitzchak, people were murmuring… Immediately the features of Yitzchak’s face were transformed and became like Avraham’s; then all opened and said, “Avraham begot Yitzchak.”
Custom (Bavli — resemblance proves paternity)
Heaven caused Yitzchak’s face to resemble Avraham’s so that all would acknowledge “Avraham begot Yitzchak.”