Abd al-Rahman bin Abdullah bin Ka'b bin Malik (may Allah have mercy on him) says that Sayyiduna Ka'b bin Malik (may Allah be pleased with him) came to the Messenger of Allah (peace and blessings be upon him) when Allah Almighty revealed something concerning poetry, and said: You know what Allah Almighty has revealed concerning poetry; what is your view? He (peace and blessings be upon him) said: "The believer strives (in jihad) with his sword and his tongue."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21109