كتاب الطهارة
2
The Book of Purification
(34)
Chapter: The evidence that urine is impure and the obligation to take precautions concerning it
(34)
باب الدَّلِيلِ عَلَى نَجَاسَةِ الْبَوْلِ وَوُجُوبِ الاِسْتِبْرَاءِ مِنْهُ ‏‏

Ibn Abbas reported:

The Messenger of Allah (ﷺ) happened to pass by two graves and said: They (their occupants) are being tormented, but they are not tormented for a grievous sin. One of them carried tales and the other did not keep himself safe from being defiled by urine. He then called for a fresh twig and split it into two parts, and planted them on each grave and then said: Perhaps, their punishment way be mitigated as long as these twigs remain fresh.
وَحَدَّثَنَا أَبُو سَعِيدٍ الأَشَجُّ، وَأَبُو كُرَيْبٍ مُحَمَّدُ بْنُ الْعَلاَءِ وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ قَالَ إِسْحَاقُ أَخْبَرَنَا وَقَالَ الآخَرَانِ، حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا الأَعْمَشُ، قَالَ سَمِعْتُ مُجَاهِدًا، يُحَدِّثُ عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ مَرَّ رَسُولُ اللَّهِ صلى الله عليه وسلم عَلَى قَبْرَيْنِ فَقَالَ ‏"‏ أَمَا إِنَّهُمَا لَيُعَذَّبَانِ وَمَا يُعَذَّبَانِ فِي كَبِيرٍ أَمَّا أَحَدُهُمَا فَكَانَ يَمْشِي بِالنَّمِيمَةِ وَأَمَّا الآخَرُ فَكَانَ لاَ يَسْتَتِرُ مِنْ بَوْلِهِ ‏"‏ ‏.‏ قَالَ فَدَعَا بِعَسِيبٍ رَطْبٍ فَشَقَّهُ بِاثْنَيْنِ ثُمَّ غَرَسَ عَلَى هَذَا وَاحِدًا وَعَلَى هَذَا وَاحِدًا ثُمَّ قَالَ ‏"‏ لَعَلَّهُ أَنْ يُخَفَّفَ عَنْهُمَا مَا لَمْ يَيْبَسَا ‏"‏ ‏.‏
Reference : Sahih Muslim 292a
In-book reference : Book 2, Hadith 143
USC-MSA web (English) reference : Book 2, Hadith 575
  (deprecated numbering scheme)