From Sahih Muslim (Hadith 677):
Ibn Abbas reported:The Messenger of Allah (ﷺ) happened to pass by two graves and said: They (their occupants) are being tormented, but they are not tormented for a grievous sin. One of them carried tales and the other did not keep himself safe from being defiled by urine. He then called for a fresh twig and split it into two parts, and planted them on each grave and then said: Perhaps, their punishment way be mitigated as long as these twigs remain fresh
Reference: Muslim 677