וְזֶה־לְּךָ֥ הָא֖וֹת מֵאֵ֣ת יְהוָ֑ה אֲשֶׁר֙ יַעֲשֶׂ֣ה יְהוָ֔ה אֶת־הַדָּבָ֥ר הַזֶּ֖ה אֲשֶׁ֥ר דִּבֵּֽר׃
Das sei dir das Zeichen vom Herrn, dass der Herr das tun wird, was er gesprochen.
Rashi on Isaiah
And this is your sign That you shall be cured, and that your days shall be increased, as is explained below (v. 22), and in Kings (2 20:8) he asked, “What is the sign that I will go up?”