Genesis
Chapters 21:1-4
New International Version
1Now the Lord was gracious to Sarah as he had said, and the Lord did for Sarah what he had promised.
2Sarah became pregnant and bore a son to Abraham in his old age, at the very time God had promised him.
3Abraham gave the name Isaac to the son Sarah bore him.
4When his son Isaac was eight days old, Abraham circumcised him, as God commanded him.