וְזֶה־לְּךָ֥ הָא֖וֹת מֵאֵ֣ת יְהוָ֑ה אֲשֶׁר֙ יַעֲשֶׂ֣ה יְהוָ֔ה אֶת־הַדָּבָ֥ר הַזֶּ֖ה אֲשֶׁ֥ר דִּבֵּֽר׃
And this shall be the sign unto thee from the LORD, that the LORD will do this thing that He hath spoken:
Read midrashic interpretation of Isaiah 38:7: and classical rabbinic exegesis.