Targum Pseudo-Jonathan on Genesis 17:17 is the mirror image of Sarah's later laugh at the tent door. Abraham falls on his face. He does not argue out loud. He laughs — wondered, the Aramaic says politely — in his heart. And the question he cannot quite ask the Lord is this: shall the son of a hundred years have progeny, and Sarah, the daughter of ninety years, bear a child?
The Hebrew tzachak, the root of the coming name Isaac, hovers over the verse. The child who will carry joy in his name has been prefigured by laughter even before his conception. First his father laughs. Then his mother will laugh (Genesis 18:12). Then he will be born, and the text will say all who hear will laugh with me (Genesis 21:6). A covenant of laughter runs underneath a covenant of flesh.
The Maggid notices the grace of it. Abraham's laugh is not punished. Sarah's will be questioned, but not punished either. The Lord apparently takes no offense when faithful people laugh in their hearts at His improbable promises. Ninety-nine years of walking and cutting and waiting, and still the best response Abraham can manage, face down in the dirt, is a private chuckle (Genesis 17:17). And the Lord lets him keep it. The covenant is strong enough to hold a patriarch's disbelief.