Musnad Ahmad 25:143

Abu Mu’awiyah and Waki’—the meaning is the same—said: Al-A’mash narrated to us, from Mujahid—Waki’ said: I heard Mujahid narrating from Tawus, from Ibn Abbas, who 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 used not to protect himself from urine"—Waki’ said: from his urine—"and as for the other, he used to walk about with slander." Then he took a palm branch, split it into two halves, and planted one in each grave. They said: "O Messenger of Allah, why did you do this?" He said: "Perhaps it may be lightened for them as long as they do not dry out"—Waki’ said: "as long as they do not dry out." *** Husayn narrated to us, Shayban narrated to us, from Mansur, from Mujahid, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by one of the gardens of Medina and heard the sound of two people being punished in their graves, and he mentioned it, and said: "Until they dry out, or: as long as they do not dry out."