Genesis 17:11-13

11 You shall circumcise the flesh of your foreskins, and it shall be a sign of the covenant between me and you.
12 Throughout your generations every male among you shall be circumcised when he is eight days old, including the slave born in your house and the one bought with your money from any foreigner who is not of your offspring.
13 Both the slave born in your house and the one bought with your money must be circumcised. So shall my covenant be in your flesh an everlasting covenant.
New Revised Standard Version Bible, copyright 1989, Division of Christian Education of the National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved.