C. F. Keil and Franz Delitzsch on 1 Kings 13:3

1Ki 13:3

In confirmation of his word the prophet added a miracle (מופת, τέρας, portentum, see at Exo 4:21): “this is the sign that the Lord hath spoken (through me): behold the altar will be rent in pieces, and the ashes upon it will be poured out.” דּשׁן is the ashes of the fat of the sacrificial animals. The pouring out of the sacrificial ashes in consequence of the breaking upon of the altar was a penal sign, which indicated, along with the destruction of the altar, the desecration of the sacrificial service performed upon it.