Sahih Bukhari Volume 001, Book 004, Hadith Number 217.
Narated By Ibn 'Abbas : The Prophet once passed by two graves and said, "These two persons are being tortured not for a major sin (to avoid). One of them never saved himself from being soiled with his urine, while the other used to go about with calumnies(to make enmity between friends)." The Prophet then took a green leaf of a date-palm tree, split it into (pieces) and fixed one on each grave. They said, "O Allah's Apostle! Why have you done so?" He replied, "I hope that their punishment might be lessened till these (the pieces of the leaf) become dry." (See the foot-note of Hadith 215).