John Gill 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)

And he gave a sign the same day, saying, this is the sign
which the Lord has spoken
As a confirmation of the truth of what the prophet had said, and for the proof of his being a true prophet of the Lord:

behold, the altar shall be rent ;
of itself, by an invisible hand, by the power of God:

and the ashes that are upon it shall be poured out ;
signifying it should be entirely demolished, not only shaken but destroyed, so as not to be capable of retaining the ashes on it; this was an emblem of the utter abolition of idolatry here in future times.