He cried with a loud voice, the effect of his passion, both for his own guilt and shame, and for the prophet's approaching misery, and his unhappy influence both in procuring and in denouncing of it. The mouth, i.e. the word of command coming out of his mouth; a metonymy of the cause for the effect.
Related Commentaries of 1 Kings 13:21
1 Kings 13:21
21 And he cried unto the man of God that came from Judah, saying, Thus saith the LORD, Forasmuch as thou hast disobeyed the mouth of the LORD, and hast not kept the commandment which the LORD thy God commanded thee,