It is narrated from Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to order us to level the graves.
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15900
Abu Ali al-Hamdani narrates that he saw Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) order that the graves of the Muslims in the land of the Romans be levelled, and they were levelled. He said: "I heard the Prophet (peace and blessings be upon him) say: 'Level your graves.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15901
Thumamah bin Shufayy (may Allah have mercy on him) narrates: We were with Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) in the land of the Romans when a companion of ours died, so Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) ordered that his grave be levelled. He said: "I heard the Messenger of Allah (peace and blessings be upon him) say it, ordering that graves be levelled."
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15902
Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When you bury (the dead), level the graves with the surface of the ground."
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15903