Genesis 17:8

5 No longer will your name be Abram, but Abraham, for I have made you the father of a number of nations.

6 I will make you very fertile, so that nations will come from you and kings will be your offspring.

7 And I will make between me and you and your seed after you through all generations, an eternal agreement to be a God to you and to your seed after you.

8 And to you and to your seed after you, I will give the land in which you are living, all the land of Canaan for an eternal heritage; and I will be their God.

9 And God said to Abraham, On your side, you are to keep the agreement, you and your seed after you through all generations.

10 And this is the agreement which you are to keep with me, you and your seed after you: every male among you is to undergo circumcision.

11 In the flesh of your private parts you are to undergo it, as a mark of the agreement between me and you.