Albert Barnes Commentary Isaiah 37:21

"Then Isaiah the son of Amoz sent unto Hezekiah, saying, Thus saith Jehovah, the God of Israel, Whereas thou hast prayed to me against Sennacherib king of Assyria," — Isaiah 37:21 (ASV)

Whereas you have prayed - Because you have come to me instead of relying on your own resources and strength. In 2 Kings 19:20, it is, That which thou hast prayed to me against Sennacherib, king of Assyria, I have heard.