Musnad Ahmad 1118:1

Muhammad ibn Ubayd narrated to us, Muhammad ibn Ishaq narrated to us, from Thumamah, who said: We went out with Fudalah ibn Ubayd to the land of the Byzantines, and he was a governor for Mu'awiyah at the mountain pass. A cousin of ours was struck [and died], so Fudalah prayed over him and stood by his grave until he had concealed him. When we had leveled his grave over him, he said: "Level it," for the Messenger of Allah (peace and blessings of Allah be upon him) used to "command us to level the graves."