John Gill Commentary 2 Kings 20:9

"And Isaiah said, This shall be the sign unto thee from Jehovah, that Jehovah will do the thing that he hath spoken: shall the shadow go forward ten steps, or go back ten steps?" — 2 Kings 20:9 (ASV)

And Isaiah said, this sign shall you have of the Lord, that the Lord will do the thing that he has spoken,.... Cure him of his disorder, so that he should be able to go to the temple on the third day:

shall the shadow go forward ten degrees, or go back ten degrees? that is, the shadow of the sun on a dial plate; it was left to his option to choose which he would, as the confirming sign of his recovery.