Bereshis 17:12-13

And he that is eight days old shall be circumcised among you, every zachar in your dorot, he that is born in the bais, or bought with kesef of any foreigner, which is not of thy zera. He that is born in thy bais, and he that is bought with thy kesef, must be circumcised; and My Brit shall be in your basar for a Brit Olam.