Sahih Bukhari Volume 1, Book 4, Hadith 218

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ خَازِمٍ، قَالَ حَدَّثَنَا الأَعْمَشُ، عَنْ مُجَاهِدٍ، عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ مَرَّ النَّبِيُّ صلى الله عليه وسلم بِقَبْرَيْنِ فَقَالَ ‏"‏ إِنَّهُمَا لَيُعَذَّبَانِ، وَمَا يُعَذَّبَانِ فِي كَبِيرٍ أَمَّا أَحَدُهُمَا فَكَانَ لاَ يَسْتَتِرُ مِنَ الْبَوْلِ، وَأَمَّا الآخَرُ فَكَانَ يَمْشِي بِالنَّمِيمَةِ ‏"‏‏.‏ ثُمَّ أَخَذَ جَرِيدَةً رَطْبَةً، فَشَقَّهَا نِصْفَيْنِ، فَغَرَزَ فِي كُلِّ قَبْرٍ وَاحِدَةً‏.‏ قَالُوا يَا رَسُولَ اللَّهِ، لِمَ فَعَلْتَ هَذَا قَالَ ‏"‏ لَعَلَّهُ يُخَفَّفُ عَنْهُمَا مَا لَمْ يَيْبَسَا ‏"‏‏.‏ قَالَ ابْنُ الْمُثَنَّى وَحَدَّثَنَا وَكِيعٌ قَالَ حَدَّثَنَا الأَعْمَشُ قَالَ سَمِعْتُ مُجَاهِدًا مِثْلَهُ ‏"‏ يَسْتَتِرُ مِنْ بَوْلِهِ ‏"

Narrated 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).