Genesis 17:1-14 (ESV)
When Abram was ninety-nine years old the Lord appeared to Abram and said to him, “I am God Almighty; walk before me, and be blameless, 2 that I may make my covenant between me and you, and may multiply you greatly.” 3 Then Abram fell on his face. And God said to him, 4 “Behold, my covenant is with you, and you shall be the father of a multitude of nations. 5 No longer shall your name be called Abram, but your name shall be Abraham, for I have made you the father of a multitude of nations. 6 I will make you exceedingly fruitful, and I will make you into nations, and kings shall come from you. 7 And I will establish my covenant between me and you and your offspring after you throughout their generations for an everlasting covenant, to be God to you and to your offspring after you. 8 And I will give to you and to your offspring after you the land of your sojournings, all the land of Canaan, for an everlasting possession, and I will be their God.”
9 And God said to Abraham, “As for you, you shall keep my covenant, you and your offspring after you throughout their generations. 10 This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. 11 You shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between me and you. 12 He who is eight days old among you shall be circumcised. Every male throughout your generations, whether born in your house or bought with your money from any foreigner who is not of your offspring, 13 both he who is born in your house and he who is bought with your money, shall surely be circumcised. So shall my covenant be in your flesh an everlasting covenant. 14 Any uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant.”