“A sword’s wound you can dress, and it heals; but what the tongue wounds remains for all time.”

Al-Jahiz quotes this line from an unnamed poet in al-Bayan wa’t-Tabyin, right after Jarir’s boast that his sword is less deadly than his tongue. A few pages earlier he gives an older form of the idea, from Imru’ al-Qays: “and the wound of the tongue is like the wound of the hand”; later poets made it the worse of the two. The Sunnah explains why: a person can say a word he thinks nothing of and fall by it further than he imagines.