Genesis 21:1-5

Then the LORD took note of Sarah as He had said, and the LORD did for Sarah as He had promised. So Sarah conceived and bore a son to Abraham in his old age, at the appointed time of which God had spoken to him. Abraham called the name of his son who was born to him, whom Sarah bore to him, Isaac. Then Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. Now Abraham was one hundred years old when his son Isaac was born to him.