Genesis 21:1-5

And the LORD visited Sarah as he had said, and the LORD did unto Sarah as he had spoken. For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac. And Abraham circumcised his son Isaac being eight days old, as God had commanded him. And Abraham was an hundred years old, when his son Isaac was born unto him.