Albert Barnes 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 - That is, a sign, or proof that God would do what He had promised, and that Hezekiah would recover and be permitted to go again to the temple of the Lord (Isaiah 38:22; 2 Kings 20:8). On the meaning of the word ‘sign,’ see (Isaiah 7:11), note; (Isaiah 7:14), note; compare the note at (Isaiah 37:30). The promise was, that he should be permitted to go to the temple in three days (2 Kings 20:5).