From Sahih Bukhari (Hadith 6055):
Narrated Ibn `Abbas:Once the Prophet (ﷺ) went through the grave-yards of Medina and heard the voices of two humans who were being tortured in their graves. The Prophet (ﷺ) said, "They are being punished, but they are not being punished because of a major sin, yet their sins are great. One of them used not to save himself from (being soiled with) the urine, and the other used to go about with calumnies (Namima)." Then the Prophet asked for a green palm tree leaf and split it into two pieces and placed one piece on each grave, saying, "I hope that their punishment may be abated as long as these pieces of the leaf are not dried
Reference: Bukhari 6055