Musannaf Ibn Abi Shayba 6:1244

Waki’ narrated to us, from al-Aswad ibn Shayban, who said: Bahr ibn Murar narrated to me, from his grandfather 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: "Indeed, they are being punished. Who will bring me a palm frond?" I and another man raced, and I outpaced him, and we brought it to him. He said: "So he split it from its head, and he planted one on this [grave] and one on this [grave]." And he said: "Perhaps it will be lightened for them as long as there remains in them any moisture." They were being punished for backbiting and [not protecting themselves from] urine.