Sahih Bukhari Volume 2, Book 23, Hadith 1379

حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ مُجَاهِدٍ، عَنْ طَاوُسٍ، قَالَ ابْنُ عَبَّاسٍ ـ رضى الله عنهما ـ مَرَّ النَّبِيُّ صلى الله عليه وسلم عَلَى قَبْرَيْنِ فَقَالَ ‏"‏ إِنَّهُمَا لَيُعَذَّبَانِ، وَمَا يُعَذَّبَانِ مِنْ كَبِيرٍ ـ ثُمَّ قَالَ ـ بَلَى أَمَّا أَحَدُهُمَا فَكَانَ يَسْعَى بِالنَّمِيمَةِ، وَأَمَّا أَحَدُهُمَا فَكَانَ لاَ يَسْتَتِرُ مِنْ بَوْلِهِ ‏"‏‏.‏ قَالَ ثُمَّ أَخَذَ عُودًا رَطْبًا فَكَسَرَهُ بِاثْنَتَيْنِ ثُمَّ غَرَزَ كُلَّ وَاحِدٍ مِنْهُمَا عَلَى قَبْرٍ، ثُمَّ قَالَ ‏"‏ لَعَلَّهُ يُخَفَّفُ عَنْهُمَا مَا لَمْ يَيْبَسَا ‏"‏‏

Narrated By Ibn Abbas : 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."