Albert Barnes Commentary 1 Kings 20:38

"So the prophet departed, and waited for the king by the way, and disguised himself with his headband over his eyes." — 1 Kings 20:38 (ASV)

Ashes - Rather, “a bandage” (and in 1 Kings 20:41). The purpose of the wound and bandage was twofold. Partly, it was to prevent Ahab from recognizing the prophet’s face; partly, to induce him to believe that the man had really been engaged in the recent war.