Musnad Ahmad 175:22

Abu al-Yaman narrated to us, he said: Shu'ayb informed us, from al-Zuhri, he said: 'Abd al-Rahman ibn 'Abd Allah ibn Ka'b ibn Malik narrated to me, that Ka'b ibn Malik, when Allah, Blessed and Exalted is He, revealed what He revealed regarding poetry, came to the Prophet, peace and blessings of Allah be upon him, and said: "Allah, Blessed and Exalted is He, has revealed regarding poetry what you know, so how do you view it?" The Prophet, peace and blessings of Allah be upon him, said: "Indeed, the believer strives with his sword and his tongue."