Sahih Bukhari, Chapter: Ablutions (Wudu')

Once the Prophet, while passing through one of the grave-yards of Medina or Mecca heard the voices of two persons who were being tortured in their graves. The Prophet said, "These two persons are being tortured not for a major sin (to avoid)." The Prophet then added, "Yes! (they are being tortured for a major sin). Indeed, one of them never saved himself from being soiled with his urine while the other used to go about with calumnies (to make enmiy between friends). The Prophet then asked for a green leaf of a date-palm tree, broke it into two pieces and put one on each grave. On being asked why he had done so, he replied, "I hope that their torture might be lessened, till these get dried."