John Gill Commentary 1 Kings 20:13

"And, behold, a prophet came near unto Ahab king of Israel, and said, Thus saith Jehovah, Hast thou seen all this great multitude? behold, I will deliver it into thy hand this day; and thou shalt know that I am Jehovah." — 1 Kings 20:13 (ASV)

And, behold, there came a prophet unto Ahab king of Israel ,
&c.] Supposed by the Jewish writers to be Micaiah the son of Imlah, (1 Kings 22:9) one of those perhaps that Obadiah hid in the cave, and now is sent out publicly on an errand to Ahab:

saying, thus saith the Lord, have you seen all this great multitude ?
considered what a vast number Benhadad's army consisted of:

behold, I will deliver it into your hand this day ;
as great as it is:

and you shall know that I am the Lord ;
and not Baal, who can save by few as well as by many, and from a great multitude.