Musnad Ahmad 605:1

Yazid ibn Harun narrated to us, Al-Awwam informed us, Abd al-Jabbar al-Khawlani narrated to us, he said: A man from the companions of the Prophet, may God bless him and grant him peace, entered the mosque, and behold, Ka'b was telling stories. He said: "Who is this?" They said: "Ka'b, telling stories." He said: "I heard the Messenger of God, may God bless him and grant him peace, say: 'No one tells stories except an amir (leader), or one who is commanded (by the leader), or a show-off (arrogant person).'" He said: That reached Ka'b, and he was not seen telling stories after that.