John Gill Commentary Isaiah 38:7

"And this shall be the sign unto thee from Jehovah, that Jehovah will do this thing that he hath spoken:" — Isaiah 38:7 (ASV)

And this shall be a sign unto thee from the Lord
It seems Hezekiah asked, and he was asked which he would choose, whether the shadow on the sundial should go forward or backward ten degrees, and he chose the latter, (2 Kings 20:8–11) , which was a token confirming and assuring that the Lord will do this thing that he has spoken ;
recover Hezekiah from his sickness, so that on the third day he should go up to the temple; have fifteen years added to his days; and the city of Jerusalem protected from the attempts of the Assyrian monarch.