Genesis 21:3

And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac. The emphasis is again upon the fact that this was the son of promise, that he was the son of Abraham, not by a servant, but by Sarah, his wife. He complied with God's command in giving his son the name Isaac (he that laughs), Genesis 17:19. As the joyous laughter of Abraham had been caused by the great contrast between the idea and the reality, so the birth was a miracle of God's mercy, whence the son should always be an object of joyful and grateful contemplation.