2 Kings 19

"Then Isaiah the son of Amoz sent to Hezekiah, saying, “Thus says the Lord God of Israel: ‘Because you have prayed to Me against Sennacherib king of Assyria, I have heard.’" v.20

Read chapter 19

This is the first we hear of Isaiah the prophet in scripture.  The book of Isaiah is well later in the Old Testament, but he prophesied over many years and among many kings. We know from the book of Isaiah of the time he prophesied,  "The vision of Isaiah the son of Amoz, which he saw concerning Judah and Jerusalem in the days of Uzziah, Jotham, Ahaz, and Hezekiah, kings of Judah" (1:1).    Isaiah is not present here, and may not have known of the prayer, but "in a sentence, Hezekiah’s prayer is answered, and then the prophet, in Jehovah’s name, bursts into a wonderful song of triumph over the defeated invader. ‘I have heard.’ That is enough. Hezekiah’s prayer has, as it were, fired the fuse or pulled the trigger, and the explosion follows, and the shot is sped. ‘Whereas thou hast prayed, . . . I have heard,’ is ever true, and God’s hearing is God’s acting in answer" (MacLaren's Exposition).  Recall a time you have prayed and it's been answered by someone that didn't even know your prayer?