Genesis 17:12

17:12 Throughout your generations every male among you who is eight days old must be circumcised, whether born in your house or bought with money from any foreigner who is not one of your descendants.

Genesis 21:1-4

The Birth of Isaac

21:1 The Lord visited Sarah just as he had said he would and did for Sarah what he had promised. 21:2 So Sarah became pregnant and bore Abraham a son in his old age at the appointed time that God had told him. 21:3 Abraham named his son – whom Sarah bore to him – Isaac. 21:4 When his son Isaac was eight days old, Abraham circumcised him just as God had commanded him to do.