Genesis 21: The Birth of Isaac

¹ The LORD came to Sarah as He had said, and the LORD did for Sarah what He had promised. ² Sarah became pregnant and bore a son to Abraham in his old age, at the appointed time God had told him.

 God did just as He had promised, on the exact day He promised. Sarah gave birth to Abraham's son, and the child was named Isaac. Abraham circumcised Isaac when he was eight days old, just as the Lord commanded. Verse 5 says that Abraham was 100 years old when Isaac was born.