Genesis 17:21

“Is anything too difficult for Yahweh? At the appointed time I will return to you, at this time next year, and Sarah will have a son.”

Now Yahweh visited Sarah as He had said, and Yahweh 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.

And Abraham called the name of his son who was born to him, whom Sarah bore to him, Isaac.

“For this reason Moses has given you circumcision (not because it is from Moses, but from the fathers), and on the Sabbath you circumcise a man.

“And He gave him the covenant of circumcision; and so Abraham was the father of Isaac, and circumcised him on the eighth day; and Isaac was the father of Jacob, and Jacob of the twelve patriarchs.

But the son by the servant-woman had been born according to the flesh, while the son by the free woman through the promise.