Musnad Ahmad 1118:25

Al-Hasan ibn Musa narrated to us, he said: Ibn Lahi'ah narrated to us, he said: Yazid ibn Abi Habib narrated to us, that Abu Ali al-Hamdani informed him that he saw Fadalah ibn 'Ubayd order the graves of the Muslims, so they were leveled with the ground in the land of the Romans. And he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Level your graves with the ground."