Genesis 17:9-14

Further, God said to Abraham, “As for you [your part of the agreement], you shall keep and faithfully obey [the terms of] My covenant, you and your descendants after you throughout their generations. This is [the sign of] My covenant, which you shall keep and faithfully obey, between Me and you and your descendants after you: Every male among you shall be circumcised. And you shall be circumcised in the flesh of your foreskins, and it shall be the sign (symbol, memorial) of the covenant between Me and you. Every male among you who is eight days old shall be circumcised throughout your generations, [including] a servant whether born in the house or one who is purchased with [your] money from any foreigner, who is not of your descendants. A servant who is born in your house or one who is purchased with your money must be circumcised; and [the sign of] My covenant shall be in your flesh for an everlasting covenant. And the male who is not circumcised in the flesh of his foreskin, that person shall be cut off from his people; he has broken My covenant.”