'Dust on His Head' in the Bible

On the third day a man came [unexpectedly] from Saul’s camp with his clothes torn and dust on his head [as in mourning]. When he came to David, he bowed to the ground and lay himself face down [in an act of great respect and submission].

When David came to the summit where he used to worship God, Hushai the Archite was there to meet him with his robe torn and dust on his head.