And Yahweh visited Sarah as he had said. And Yahweh did to Sarah as he had promised.
References for Genesis 21:1
And she conceived, and Sarah bore to Abraham a son in his old age at the appointed time that God had told him.
And Abraham called the name of his son who was born to him, whom Sarah bore to him, Isaac.
And Abraham circumcised Isaac his son [when he was] eight days old, as God had commanded him.
And Abraham [was] one hundred years old when Isaac his son was born to him.
And Sarah said, "God has made laughter for me; all who hear will laugh for me."
And she said, "Who would announce to Abraham [that] Sarah would nurse children? Yet I have borne a son [to Abraham] in his old age."