Albert Barnes Commentary 1 Kings 13:21

"and he cried unto the man of God that came from Judah, saying, Thus saith Jehovah, Forasmuch as thou hast been disobedient unto the mouth of Jehovah, and hast not kept the commandment which Jehovah thy God commanded thee," — 1 Kings 13:21 (ASV)

Since you have disobeyed the mouth of the Lord — He should not have allowed himself to be persuaded. He should have felt that his obedience was being tested and, before considering himself released from his duty, should have required evidence that was the same as, or as strong as, the evidence on which he had received the original command.

Disobedience to God's specific commands was a sin that was very important to punish notably at this time, since it was exactly the sin of Jeroboam and his followers.