Sforno on Genesis 21:9/ספורנו על בראשית כ״א:ט׳

1את בן הגר המצרית, she assumed that the reason Ishmael had made disparaging remarks about Yitzchok was due to his mother putting him up to it, or to the Egyptian genes of his mother coming into play here. We have a saying in Sukkah 56 that the prattle of children in public reflects either what they picked up from their father or what they picked up from their mother. In the case of Ishmael, he certainly did not overhear his father make such disparaging remarks.

2מצחק, making fun of the big party Avraham had given to mark the weaning of Yitzchok, He claimed that surely Sarah must have become pregnant from Avimelech. The reason he had not made such remarks already at the time when Yitzchok had been born, was because he had only overheard wicked gossip about this at a later stage, and now he repeated what he had heard.