Genesis 21:1-4 King James Version (KJV)
21 And the Lord visited Sarah as he had said, and the Lord did unto Sarah as he had spoken.
2 For 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 being eight days old, as God had commanded him.