What sign did God give Hezekiah to tell him he would live another 15 years?

He moved a shadow back ten degrees.

Go, and say to Hezekiah, Thus saith the LORD, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will add unto thy days fifteen years. ...
And this shall be a sign unto thee from the LORD, that the LORD will do this thing that he hath spoken.
Behold, I will bring again the shadow of the degrees, which is gone down in the sun dial of Ahaz, ten degrees backward. So the sun returned ten degrees. Isaiah 38:5-8