And a certain man of the sons of the prophets said unto his neighbour in the word of the LORD, Smite me, I pray thee. And the man refused to smite him. In the word of the Lord - By the word or command of the Lord; that is, God has commanded thee to smite me. Refusing to do it, this man forfeited his life, as we are informed in the next verse.
By this emblematical action he intended to inform Ahab that, as the man forfeited his life who refused to smite him when he had the Lord's command to do it; so he (Ahab) had forfeited his life, because he did not smite Ben-hadad when he had him in his power.