Genesis 21:4

4 And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him.

Genesis 21:4 In-Context

2 And Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.
3 And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.
4 And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him.
5 And Abraham was a hundred years old, when his son Isaac was born unto him.
6 And Sarah said, God hath made me to laugh. Every one that heareth will laugh with me.
The American Standard Version is in the public domain.