John Gill Commentary 1 Kings 22:14

"And Micaiah said, As Jehovah liveth, what Jehovah saith unto me, that will I speak." — 1 Kings 22:14 (ASV)

And Micaiah said, as the Lord lives
He swore by the living God, for the confirmation of what he was about to say:

what the Lord says to me, that will I speak ;
truly and faithfully, keeping nothing back, nor adding anything, whether it be good or evil, pleasing or displeasing; it looks as if as yet he had no instruction from the Lord what to say, and yet the vision he later declares seems to have been had by him before, (1 Kings 22:17 1 Kings 22:19) .