Genesis 21:9

Ishmael had no doubt been regarded as Abraham's heir until the birth of Isaac. The change in his prospects may account for his conduct, which St. Paul uses to illustrate the persecution of the Christians by the Jews (Galatians 4:29). Proud of their natural descent as children of Abraham, the Jews scorned the idea that God could regard others as His spiritual children and allow them to share in their privileges and blessings: see Galatians 3:4. The story affords painful evidence of the jealousies and unhappiness caused by polygamy.