Albert Barnes Commentary 1 Kings 13:1

"And, behold, there came a man of God out of Judah by the word of Jehovah unto Beth-el: and Jeroboam was standing by the altar to burn incense." — 1 Kings 13:1 (ASV)

Rather, “in the word of the Lord.” The meaning seems to be that the prophet was not merely commanded to come, but that he came in the strength and power of God’s word as a divinely inspired messenger. (Compare to 1 Kings 13:2, 1 Kings 13:5, and 1 Kings 13:32.)

By the altar means “on the altar”; that is, on the ledge or platform halfway up the altar, on which the officiating priest always stood to sacrifice. (Compare to the note on 1 Kings 12:32.)