Isaac is born (Gen. 21:1-6), and when the time comes for him to be weaned, Abraham makes a great feast, and Hagar and Ishmael attend the celebration. When Sara sees Ishmael, who would have been about 15 years old, playing with his little brother, she demands that Abraham “cast out this slave woman with her son, for the son of this slave woman shall not inherit along with my son Isaac (Gen. 21:10b).   

Abraham is distressed by Sarah’s demand, as we discover with the first words God speaks to him, “Do not be distressed because of the boy and because of your slave woman” (Gen 21:12b), and God then promises to make a great nation of Ishmael also, because he is the offspring of Abraham.      

The disturbing result, then, is that Abraham takes Hagar and Ishmael out to the desert, gives them some bread and water, and abandons them there.

So Abraham rose early in the morning, and took bread and a skin of water, and gave it to Hagar, putting it on her shoulder, along with the child, and sent her away. And she departed, and wandered about in the wilderness of Beer-sheba. Gen. 21:14.

What follows in this story is devastating and moving—Hagar and Ishmael run out of water, and the boy’s cries are too heart-rending for her, so she sets him under a tree and moves far enough away so she doesn’t have to see to her son die:

When the water in the skin was gone, she cast the child under one of the bushes. Then she went and sat down opposite him a good way off, about the distance of a bowshot; for she said, ‘Do not let me look on the death of the child.’ Gen. 21: 15-16

What happens next is what we should expect but which still comes as a surprising relief: Hagar lifts up her voice and weeps; God hears and answers her prayers.

Do not be afraid; for God has heard the voice of the boy where he is. Come, lift up the boy and hold him fast with your hand, for I will make a great nation of him.’ Then God opened her eyes, and she saw a well of water. She went, and filled the skin with water, and gave the boy a drink. Gen  21:18-18.  

This part of the story of Ishmael ends with a hint at the future:

God was with the boy, and he grew up; he lived in the wilderness, and became an expert with the bow. He lived in the wilderness of Paran; and his mother got a wife for him from the land of Egypt.  Gen: 21:20-21

The fulfillment of God’s promise to Ishmael appears two times in Genesis. A genealogy of Ishmael descendants appears in Gen. 25:12-18.  And then later we find Ishmael stature rises so that a tribe of people are named after him—Ishmaelites.