Albert Barnes Commentary 1 Kings 22:8

"And the king of Israel said unto Jehoshaphat, there is yet one man by whom we may inquire of Jehovah, Micaiah the son of Imlah: but I hate him; for he doth not prophesy good concerning me, but evil. And Jehoshaphat said, Let not the king say so." — 1 Kings 22:8 (ASV)

There is still one man, Micaiah — Elijah, it appears, had withdrawn again after the events of the last chapter, and there was no known prophet of Yahweh within reach of Samaria except Micaiah.

He doth not prophesy good concerning me, but evil — Whether the tradition mentioned in the note for 1 Kings 20:41 is true or not, it is certain that Ahab had imprisoned him (1 Kings 22:26), and it is probable that the imprisonment was on account of threatening prophecies. Ahab suggests to Jehoshaphat that Micaiah is one who allows his private feelings to determine the utterances he delivers as if from Yahweh. This explains the force of Jehoshaphat’s answer, Let not the king say so; that is, the king should not assume that a prophet would be guilty of such impiety—an impiety from which even Balaam shrank (Numbers 22:18).