Albert Barnes Commentary 1 Kings 20:28

"And a man of God came near and spake unto the king of Israel, and said, Thus saith Jehovah, Because the Syrians have said, Jehovah is a god of the hills, but he is not a god of the valleys; therefore will I deliver all this great multitude into thy hand, and ye shall know that I am Jehovah." — 1 Kings 20:28 (ASV)

A man of God - This was evidently not the prophet who had spoken to Ahab the year before (1 Kings 20:13, 22). He probably lived in the neighborhood of Samaria. Now that Ahab and his army had marched out into the Trans-Jordanic territory, another prophet, likely a native of that region, announced God’s will to them.