Some time after Ishmael was born, God made the agreement of circumcision with Abraham and all the men in his family. Circumcision is cutting off part of the skin of the male flesh on boys, usually done when the boy is a baby. This special sign helped them remember the agreement that God made with Abraham:
Abram fell facedown, and God said to him, “As for me, this is my covenant with you: You will be the father of many nations. No longer will you be called Abram; your name will be Abraham, for I have made you a father of many nations. I will make you very fruitful; I will make nations of you, and kings will come from you.… This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised. You are to undergo circumcision, and it will be the sign of the covenant between me and you. Genesis 17.3-6, 10-11