Genesis 21:9 - The Popular Commentary by Paul E. Kretzmann

And Sarah saw the son of Hagar, the Egyptian, which she had born unto Abraham, mocking. It was a jeering laughter, a sneer, which Ishmael affected, perhaps as early as the festival of weaning. Unbelief, jealousy, and pride were aroused in Ishmael by the fact that Isaac was plainly the heir of the household. The mimicking, mocking, ridiculing on the part of Ishmael against Isaac could not long remain hidden from Sarah.