Genesis 21:1-5 New Revised Standard Version (NRSV)
The Birth of Isaac
21 The Lord dealt with Sarah as he had said, and the Lord did for Sarah as he had promised. 2 Sarah conceived and bore Abraham a son in his old age, at the time of which God had spoken to him. 3 Abraham gave the name Isaac to his son whom Sarah bore him. 4 And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. 5 Abraham was a hundred years old when his son Isaac was born to him.