וַיֹּ֣אמֶר יְשַׁעְיָ֗הוּ זֶה־לְּךָ֤ הָאוֹת֙ מֵאֵ֣ת יְהוָ֔ה כִּ֚י יַעֲשֶׂ֣ה יְהוָ֔ה אֶת־הַדָּבָ֖ר אֲשֶׁ֣ר דִּבֵּ֑ר הָלַ֤ךְ הַצֵּל֙ עֶ֣שֶׂר מַֽעֲל֔וֹת אִם־יָשׁ֖וּב עֶ֥שֶׂר מַעֲלֽוֹת׃
And Isaiah said: ‘This shall be the sign unto thee from the LORD, that the LORD will do the thing that He hath spoken: shall the shadow go forward ten degrees, or go back ten degrees?
Rashi
and he gave him a sign (ibid. verse 9ff.) “This is your sign, etc.” Everything is delineated there.