Genesis 17:10-13

This is the covenant that you and your descendants must keep: Each male among you must be circumcised. You must cut off the flesh of your foreskin as a sign of the covenant between me and you. From generation to generation, every male child must be circumcised on the eighth day after his birth. This applies not only to members of your family but also to the servants born in your household and the foreign-born servants whom you have purchased. All must be circumcised. Your bodies will bear the mark of my everlasting covenant.