Genesis 17:12-13

From now on when a baby boy is eight days old, you will circumcise him. This includes any boy born among your people or any who is your slave. (He would not be one of your descendants.) So circumcise every baby boy. Circumcise him whether he is born in your family or bought as a slave. Your bodies will be marked. This will show that you are part of my agreement that lasts forever.