Sunan An-Nasai, Chapter: The Book of Purification

"The Messenger of Allah (saw) 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 or 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 plit 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 as long as this does not dry out.'" (Sahih) Mansur contradicted him, he reported it from Mujahid and from Ibn Abbas, but he did not mention Tawus in it.