Musnad Ahmad 175:33

Ali ibn Bahr narrated to us, Abd al-Aziz ibn Muhammad al-Darawardi narrated to us, from Muhammad ibn Abdullah, the nephew of Ibn Shihab, from Ibn Shihab, from Abd al-Rahman ibn Abdullah ibn Ka'b, from Ka'b ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Satirize [the enemies] with poetry, for the believer strives with his self and his wealth. By the One in Whose hand is the soul of Muhammad, it is as if you are pelting them with arrows."