Musnad Ahmad 830:39

Waki’ narrated to us, al-Aswad ibn Shayban narrated to us, from Bahr ibn Murrar, from Abu Bakrah, who said: I was walking with the Prophet, peace and blessings of Allah be upon him, and he passed by two graves. He said: "Who will bring me a palm frond?" He said: So I and another man raced, and we brought a palm branch. He split it into two, and he placed one on this [grave] and one on this [grave]. Then he said: "As for it, it will lighten [the punishment] for them as long as there is any moisture in them." Then he said: "They are being punished for backbiting and [not protecting themselves from] urine."