Mishkat al-Masabih

Ibn ‘Abbas told that the Prophet came upon two graves and said, "Their occupants are being punished, but not for a great sin. One of them did not keep himself from being defiled by urine. (A version by Muslim has "did not keep himself undefiled by urine”.) The other went about spreading slander.” He then took a fresh palm branch, split it in two parts, and planted one on each grave. On being asked why he did this, he said, "Perhaps their punishment may be mitigated as long as they remain fresh.” (Bukhari and Muslim.)