בַּיָּמִ֣ים הָהֵ֔ם חָלָ֥ה יְחִזְקִיָּ֖הוּ עַד־לָמ֑וּת וַיִּתְפַּלֵּל֙ אֶל־יְהוָ֔ה וַיֹּ֣אמֶר ל֔וֹ וּמוֹפֵ֖ת נָ֥תַן לֽוֹ׃
In those days Hezekiah was sick even unto death; and he prayed unto the LORD; and He spoke unto him, and gave him a sign.
Rashi on II Chronicles
and he prayed to the Lord as is delineated in II Kings (20:3).
Rashi on II Chronicles
and he gave him a sign (ibid. verse 9ff.) “This is your sign, etc.” Everything is delineated there.