Sahih Bukhari, Chapter: Funerals (Al-Janaaiz)

The Prophet once passed by two graves and said, "They (the deceased persons in those graves) are being tortured not for a great thing to avoid." And then added, "Yes, (they are being punished for a big sin), for one of them used to go about with calumnies while the other never saved himself from being soiled with his urine." (Ibn Abbas added): Then he took a green leaf of a date-palm) and split it into two pieces and fixed one piece on each grave and said, "May their punishment be abated till these (two pieces) get dry."