Musnad Abu Ya'la al-Mawsili 109:51

Khallad ibn Aslam narrated to us, al-Nadr informed us, Abu al-Awwam Abd al-Aziz ibn Rabi' al-Bahili—and his residence was in Dar Ziyad—informed us, he said: I heard Abu al-Zubayr, and his name is Muhammad, from Jabir ibn Abdullah, who said: We were with the Prophet, may Allah bless him and grant him peace, on a journey, and he came upon two graves whose occupants were being punished. He said: "Know that they are not being punished for a major sin. As for one of them, he used to backbite people, and as for the other, he did not protect himself from his urine." Then he called for a fresh palm branch, or two branches, and broke them. Then he ordered each piece to be planted on a grave. The Messenger of Allah, may Allah bless him and grant him peace, said: "Know that their punishment will be lightened as long as they remain fresh, or as long as they do not dry out."