So the prophet departed, and waited for the king by the way, and disguised himself with ashes upon his face.

Wesley's Notes

20:38 With ashes - Or, with a cloath, or band; (as the Hebrew doctors understand the word) whereby he bound up his wound, which probably was in his face; for it was to be made in a conspicuous place, that it might be visible to Ahab and others.

Matthew Henry's Concise Commentary

20:31-43 This encouragement sinners have to repent and humble themselves before God; Have we not heard, that the God of Israel is a merciful God? Have we not found him so? That is gospel repentance, which flows from an apprehension of the mercy of God, in Christ; there is forgiveness with him. What a change is here! The most haughty in prosperity often are most abject in adversity; an evil spirit will thus affect a man in both these conditions. There are those on whom, like Ahab, success is ill bestowed; they know not how to serve either God or their generation, or even their own true interests with their prosperity: Let favour be showed to the wicked, yet will he not learn righteousness. The prophet designed to reprove Ahab by a parable. If a good prophet were punished for sparing his friend and God's when God said, Smite, of much sorer punishment should a wicked king be thought worthy, who spared his enemy and God's, when God said, Smite. Ahab went to his house, heavy and displeased, not truly penitent, or seeking to undo what he had done amiss; every way out of humour, notwithstanding his victory. Alas! many that hear the glad tidings of Christ, are busy and there till the day of salvation is gone.