Genesis 17:9-14

God said to Abraham: For your part, you and your descendants after you must keep my covenant throughout the ages. This is the covenant between me and you and your descendants after you that you must keep: every male among you shall be circumcised. Circumcise the flesh of your foreskin. That will be the sign of the covenant between me and you. Throughout the ages, every male among you, when he is eight days old, shall be circumcised, including houseborn slaves and those acquired with money from any foreigner who is not of your descendants. Yes, both the houseborn slaves and those acquired with money must be circumcised. Thus my covenant will be in your flesh as an everlasting covenant. If a male is uncircumcised, that is, if the flesh of his foreskin has not been cut away, such a one will be cut off from his people; he has broken my covenant.