Language:

Narrated Ibn `Abbas:

Allah's Messenger (ﷺ) passed by two graves and said, "Both of them (persons in the grave) are being tortured, and they are not being tortured for a major sin. This one used not to save himself from being soiled with his urine, and the other used to go about with calumnies (among the people to rouse hostilities, e.g., one goes to a person and tells him that so-and-so says about him such-and-such evil things). The Prophet (ﷺ) then asked for a green leaf of a date-palm tree, split it into two pieces and planted one on each grave and said, "It is hoped that their punishment may be abated till those two pieces of the leaf get dried." (See Hadith No 215, Vol 1).

حَدَّثَنَا يَحْيَى، حَدَّثَنَا وَكِيعٌ، عَنِ الأَعْمَشِ، قَالَ سَمِعْتُ مُجَاهِدًا، يُحَدِّثُ عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ مَرَّ رَسُولُ اللَّهِ صلى الله عليه وسلم عَلَى قَبْرَيْنِ فَقَالَ ‏"‏ إِنَّهُمَا لَيُعَذَّبَانِ، وَمَا يُعَذَّبَانِ فِي كَبِيرٍ، أَمَّا هَذَا فَكَانَ لاَ يَسْتَتِرُ مِنْ بَوْلِهِ، وَأَمَّا هَذَا فَكَانَ يَمْشِي بِالنَّمِيمَةِ ‏"‏‏.‏ ثُمَّ دَعَا بِعَسِيبٍ رَطْبٍ، فَشَقَّهُ بِاثْنَيْنِ، فَغَرَسَ عَلَى هَذَا وَاحِدًا وَعَلَى هَذَا وَاحِدًا ثُمَّ قَالَ ‏"‏ لَعَلَّهُ يُخَفَّفُ عَنْهُمَا، مَا لَمْ يَيْبَسَا ‏"‏‏.‏
Reference : Sahih al-Bukhari 6052
In-book reference : Book 78, Hadith 82
USC-MSA web (English) reference : Vol. 8, Book 73, Hadith 78
  (deprecated numbering scheme)