Albert Barnes Commentary 1 Kings 13:3

"And he gave a sign the same day, saying, This is the sign which Jehovah hath spoken: Behold, the altar shall be rent, and the ashes that are upon it shall be poured out." — 1 Kings 13:3 (ASV)

He gave a sign - A sign of this kind—an immediate prophecy to prove the divine character of a remote prophecy—had scarcely been given before this. In later history, however, such signs are not infrequent (Compare to 2 Kings 19:29; Isaiah 7:14–16).

The ashes... shall be poured out - That is, “The half-burnt remains of the offerings shall be ignominiously spilled upon the ground.”