The Man Of God

I King 13:
”And behold, there came a man of God out of Judah by the word of the Lord unto Beth-el: and Jeroboam stood by the altar to burn incense. And he cried against the altar…and when he (the man of God) left, a lion met him by the way, and slew him: and his carcass was cast in the way, and the ass stood by it, the lion also stood by the carcass.”

What was the prophet’s primary sin that led to disobedience?