Albert Barnes Commentary 2 Kings 20:8

"And Hezekiah said unto Isaiah, What shall be the sign that Jehovah will heal me, and that I shall go up unto the house of Jehovah the third day?" — 2 Kings 20:8 (ASV)

And Hezekiah said - Before his actual recovery, Hezekiah, who at first may have felt no better, asked for a "sign" that he would indeed be restored to health.

Asking for a sign is a pious or a wicked act according to the spirit in which it is done. No blame is attached to the requests of Gideon (Judges 6:17, 37, 39) or to Hezekiah's request, because they were genuine desires of the heart expressed humbly. The evil generation that sought for a sign in our Lord's time did not really want one, but made the demand contentiously, neither expecting nor wishing that it would be granted.