Musnad Ahmad 1118:3

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Thumamah ibn Shafi' al-Hamdani narrated to me, saying: We raided the land of the Byzantines, and over that army was Fadalah ibn 'Ubayd al-Ansari. He mentioned the hadith, and Fadalah said: "Level them, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, ordering the leveling of graves."