Genesis 17:15

11 You are to circumcise the flesh of your foreskin, and this will be a sign of the covenant between Me and you.

12 Generation after generation, every male must be circumcised when he is eight days old, including those born in your household and those purchased from a foreigner—even those who are not your offspring.

13 Whether they are born in your household or purchased, they must be circumcised. My covenant in your flesh will be an everlasting covenant.

14 But if any male is not circumcised, he will be cut off from his people; he has broken My covenant.”

15 Then God said to Abraham, “As for Sarai your wife, do not call her Sarai, for her name is to be Sarah.

16 And I will bless her and will surely give you a son by her. I will bless her, and she will be the mother of nations; kings of peoples will descend from her.”

17 Abraham fell facedown. Then he laughed and said to himself, “Can a child be born to a man who is a hundred years old? Can Sarah give birth at the age of ninety?”

18 And Abraham said to God, “O that Ishmael might live under Your blessing!”

19 But God replied, “Your wife Sarah will indeed bear you a son, and you are to name him Isaac. I will establish My covenant with him as an everlasting covenant for his descendants after him.

Read all of Genesis 17