Albert Barnes Commentary 1 Kings 20:33

"Now the men observed diligently, and hasted to catch whether it were his mind; and they said, Thy brother Ben-hadad. Then he said, Go ye, bring him. Then Ben-hadad came forth to him; and he caused him to come up into the chariot." — 1 Kings 20:33 (ASV)

The meaning of this verse is that Ben-hadad's men were on the watch from the moment they arrived, listening for what Ahab would say. The instant Ahab let the expression “He is my brother” slip, they seized upon it and repeated it, essentially holding him to his word and preventing him from retracting it.

By the ancient Near Eastern law of “dakheel,” anyone could place himself under another's protection at any time, whether that person was a friend or his greatest enemy. If the one asked for protection did not immediately refuse, and if even the slightest friendly words passed between them, the bond was sealed and could not be broken.

Ben-hadad’s men were watching for an opportunity to secure this “dakheel” for him. The single phrase, “He is my brother,” which they accepted on Ben-hadad's behalf, was enough to seal the bond and save the captive's life.

Having called Ben-hadad his brother, Ahab then treated him as one. He brought him up into his chariot—an act which was the greatest possible honor.