What sign did the LORD give Hezekiah that he would be healed?

Answer: The shadow went backward

Isaiah called out to the LORD, who brought the shadow back ten steps on the stairway (KJV: dial) of Ahaz.

“And Isaiah had replied, “This will be a sign to you from the LORD that He will do what He has promised: Would you like the shadow to go forward ten steps, or back ten steps?””— 2 Kings 20:9 (BSB)