Genesis 17:9-11

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.