Musannaf Ibn Abi Shayba 1:1308

Waki’ narrated to us, he said: Al-Aswad ibn Shayban narrated to us, he said: Bahr ibn Murar narrated to us, from his grandfather Abu Bakrah, he said: The Prophet, peace and blessings of Allah be upon him, passed by two graves and said: "Indeed, they are being punished, and they are not being punished for a major matter. As for one of them, he is being punished for [not protecting himself from] urine, and as for the other, he is being punished for backbiting."