John Gill 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)

And there came a man of God
The same as before, (1 Kings 20:13 1 Kings 20:22) or had come ^F16 before the little army went out to meet the Syrians; though he might go to Ahab when encamped, for his encouragement:

and spoke unto the king of Israel, and said, thus says the Lord,
because the Syrians have said, the Lord is God of the hills, but he
is not God of the valleys; (See Gill on 1 Kings 20:23).

therefore will I deliver, all this great multitude into your hand ;
not for Ahab's sake would the Lord do this, who does not appear thankful to God for the former victory, nor to be reformed from his idolatry, and the better for it, but for the honour of his own name, which had been blasphemed by the Syrians:

and you shall know that I am the Lord ;
both of hills and valleys, the omnipotent Jehovah, the only true God.

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FOOTNOTES:

• F16:
  F16 (vgyw) "accesserat", Vatablus.