Genesis 18:10

“And I will bless her, and indeed I will give you a son by her. Then I will bless her, and she shall be a mother of nations; kings of peoples will come from her.”

But God said, “No, but Sarah your wife will bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his seed after him.

“But My covenant I will establish with Isaac, whom Sarah will bear to you at this season next year.”

“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.

For this is the word of promise: “At this time I will come, and Sarah shall have a son.”

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

Do not neglect to show hospitality to strangers, for by this some have entertained angels without knowing it.