Musnad Ahmad 830:1

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Al-Aswad ibn Shayban narrated to us, Bahr ibn Murrar narrated to us, from ‘Abd al-Rahman ibn Abi Bakrah, who said: Abu Bakrah narrated to us, saying: While I was walking with the Messenger of Allah (peace and blessings of Allah be upon him) and he was holding my hand, and there was a man to his left, we suddenly came upon two graves in front of us. The Messenger of Allah (peace and blessings of Allah be upon him) said: "They are being punished, and they are not being punished for a major matter—yet indeed it is—so which of you will bring me a palm branch?" We raced, and I outpaced them, and I brought him a palm branch. He broke it into two halves, then placed a piece on this grave and a piece on that grave, and said: "It will lighten [the punishment] for them as long as they remain moist, and they are not being punished except for [not protecting themselves from] urine and backbiting."