TSK · Genesis 18:10

I will bless her, and moreover I will give you a son by her. Yes, I will bless her, and she will be a mother of nations. Kings of peoples will come from her."

But my covenant I establish with Isaac, whom Sarah will bear to you at this set time next year."

Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him.

The angel of Yahweh appeared to the woman, and said to her, "See now, you are barren, and don't bear; but you shall conceive, and bear a son.

But the angel said to him, "Don't be afraid, Zacharias, because your request has been heard, and your wife, Elizabeth, will bear you a son, and you shall call his name John.

However, the son by the handmaid was born according to the flesh, but the son by the free woman was born through promise.