Musnad Ahmad 30:2558

Muhammad ibn Ubayd narrated to us, from Yazid—meaning ibn Kaysan—from Abu Hazim, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by a grave and said, "Bring me two palm fronds." He placed one of them at his head and the other at his feet. It was said, "O Prophet of Allah, does that benefit him?" He said, "The punishment of the grave will continue to be lightened for him as long as there is moisture in them."