The primary source for the mitzvah of brit milah is found in Genesis chapter 17:
“And I will establish my covenant between me and you and your seed after you in their generations for an everlasting covenant, to be a God to you, and to your seed after you. …Such shall be the covenant between Me and you and your offspring to follow which you shall keep: every male among you shall be circumcised. You shall circumcise the flesh of your foreskin, and that shall be the sign of the covenant between Me and you.”
God instructs Abraham to circumcise himself, which he does. Abraham then circumcises all the males in his household, and when Isaac is born, he is circumcised on the eighth day of life. This sets the precedent for the
Ritual circumcision serves as an indelible sign of our eternal covenant with God and loyalty to our community