Punishment in the grave due to impurity

It is narrated on the authority of Abdullah Ibn Abbas (ra) that once the Apostle of Allah (happened to) pass by two graves, and he remarked: "The two persons who are buried in these graves are being punished, and the punishment is not owing to a sin that was a difficult matter (i.e., hard to avoid). (Both of them, on the other hand, are receiving the chastisement for a misdeed from which it was quite easy to keep away). The sin of one of them was that he never tried to save himself from being soiled with urine, and the other used to speak ill of others in their absence." The Prophet, then, took a green branch of a date palm, split it into two and fixed one piece (of it) on each grave. The Companions (thereupon) enquired: "O Apostle of Allah! Why have you done that?" "It is hoped," replied the Prophet, "that the punishment of these persons will be abated until such time as the two pieces become completely dry." (Bukhari and Muslim)