Abraham and Sarah at Their Desert Campby Ironbark

Genesis 21:9

And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking.

Ishmael openly mocked Isaac, the heir of God's promise. His conduct toward Isaac was disrespectful, marked by jealous hostility. The mocking followed Isaac's birth, which stirred jealousy in both Ishmael and Hagar. Ishmael despised Isaac because he thought Isaac was being preferred over him. Sarah saw Ishmael's turbulent disposition as a continuing source of discord and therefore asked Abraham to send Hagar and Ishmael away.