The Messenger of Allah ﷺ split a fresh palm branch into two and placed one part on each grave. He expressed hope that the punishment would be lightened while the branches remained fresh. (Sahih al-Bukhari)
Takhfif al-'adhab
A reduction in the severity of punishment.
This incident is transmitted together with the report of the two punished grave occupants. It joins the reality of punishment, the Prophet's awareness of their condition and his hope for some relief in that circumstance.