Genesis 21:9

“And Sarah saw the son of Hagar the Egyptian, etc.] That is, Ishmael, who is not expressed by name, but described by being a son of Hagar, a servant of Sarah’s, and an Egyptian woman; all which seems to be observed by way of slight, both to Hagar and her son; which she had born unto Abraham; not unto Sarah, as she proposed to herself, when she gave her maid to Abraham, (<011602>Genesis 16:2). This son of Abraham she saw mocking; either at the entertainment made at the weaning of Isaac; or rather at Isaac himself, laughing at his name, and treating him with contempt as his younger brother, and boasting that he was the firstborn, and that the inheritance belonged to him; and threatening what he…”

“Sarah saw the son of Hagar . . . mocking--Ishmael was aware of the great change in his prospects, and under the impulse of irritated or resentful feelings, in which he was probably joined by his mother, treated the young heir with derision and probably some violence (Gal 4:29).”