Genesis 17:12-13

Every male child among you must be circumcised when he is eight days old. You must continue to do this for every generation of your descendants. Circumcise every male that lives with you. Circumcise your own family. Circumcise servants that have been born in your house. Also circumcise foreign servants that you have bought with money. You must circumcise all the male servants who work for you in your house. Circumcision will be a mark in your bodies which shows that you accept my covenant. It will continue for all time.