Gen 17:12-14

12-13 From now on, your family must circumcise every baby boy when he is eight days old. You must even circumcise any man or boy you have as a slave, both those born in your homes and those you buy from foreigners. This will be a sign that my promise to you will last forever. 14 Any man who isn't circumcised hasn't kept his part of the promise and cannot be one of my people.