Therefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac.
Read Chapter 21
Ephrem The Syrian
Then Sarah, who showed no envy in any matter that concerned herself, became envious in this matter concerning her son. She was not envious of Hagar whom she had given to her husband. Since it was a matter of God’s promise, and the son of the concubine thought that he would be coheir with the son of the freewoman, Sarah said, “Cast out the slave woman and her son, because it is not just that a son of a handmaid should have any inheritance together with that son of the promise, to whom it was promised by God. It is not right that you be opposed to God and make an heir him whom God has not made an heir.”