Sahih Muslim Book 4, Hadith Number 2114.

Chapter : Commandment in regard to the levelling of the grave.

Thumama b. Shafayy reported: When we were with Fadala b. ‘Ubaid in the country of the Romans at a place (known as) Rudis, a friend of ours died. Fadala b. ‘Ubaid ordered to prepare a grave for him and then it was levelled; and then he said: I heard the Messenger of Allah (may peace be upon him) commanding (us) to level the grave.