Genesis 17:11

9And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations.

10This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.

11And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you.

12And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed.

13He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant.