Sunan an-Nasa'i 31
It was narrated that Ibn 'Abbas said:
"The Messenger of Allah (ﷺ) passed by two graves and said: 'These two are being punished, but they are not being punished for something that was difficult to avoid. As for this, he used not to take precautions to avoid (his body to clothes being soiled by) urine, and this one used to walk around spreading malicious gossip.' Then he called for a fresh palm-leaf stalk and split it in two, and placed one piece on each of the two graves. They said: 'O Messenger of Allah, why did you do that?' He said: 'Perhaps the torment will be reduced for them so long as this does not dry out.'"

Mansur Contradicted him, he reported it from Mujahid from ibn 'Abbas but he did not mention Tawus in it.
أَخْبَرَنَا هَنَّادُ بْنُ السَّرِيِّ، عَنْ وَكِيعٍ، عَنِ الأَعْمَشِ، قَالَ سَمِعْتُ مُجَاهِدًا، يُحَدِّثُ عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ مَرَّ رَسُولُ اللَّهِ صلى الله عليه وسلم عَلَى قَبْرَيْنِ فَقَالَ ‏"‏ إِنَّهُمَا يُعَذَّبَانِ وَمَا يُعَذَّبَانِ فِي كَبِيرٍ أَمَّا هَذَا فَكَانَ لاَ يَسْتَنْزِهُ مِنْ بَوْلِهِ وَأَمَّا هَذَا فَإِنَّهُ كَانَ يَمْشِي بِالنَّمِيمَةِ ‏"‏ ‏.‏ ثُمَّ دَعَا بِعَسِيبٍ رَطْبٍ فَشَقَّهُ بِاثْنَيْنِ فَغَرَسَ عَلَى هَذَا وَاحِدًا وَعَلَى هَذَا وَاحِدًا ثُمَّ قَالَ ‏"‏ لَعَلَّهُ يُخَفَّفُ عَنْهُمَا مَا لَمْ يَيْبَسَا ‏"‏ ‏.‏ خَالَفَهُ مَنْصُورٌ رَوَاهُ عَنْ مُجَاهِدٍ عَنِ ابْنِ عَبَّاسٍ وَلَمْ يَذْكُرْ طَاوُسًا ‏.‏