2 Kings 19:6
And Isaiah said unto them, Thus shall ye say to your master, Thus saith the LORD, Be not afraid of the words which thou hast heard, with which the servants of the king of Assyria have blasphemed me. And Isaiah said unto them, Thus shall ye say to your master, Thus saith the LORD, Be not afraid of the words which thou hast heard, with which the servants of the king of Assyria have blasphemed me.
God's answer through Isaiah to Hezekiah was given in response to the prayers that Hezekiah and Isaiah cried to heaven. It came after Hezekiah tore his clothes and covered himself with sackcloth and went into the house of the Lord, then sent to Isaiah asking him to pray for the remnant that was left, because Rabshakeh had been sent to reproach the living God. In that message Hezekiah was told not to be afraid of the words he had heard, with which the servants of the king of Assyria had blasphemed God.
The Assyrian claim that Israel's God could not deliver Jerusalem was not merely a threat against Judah but an attack on the living God himself. Their insults against God would not prevail, for God would answer their blasphemy by bringing judgment on the Assyrian king. The reassurance not to fear was joined to the promise that God would send a blast upon the Assyrian king, cause him to hear a rumor and return to his own land, and bring about his death by the sword there.