Abu Ali al-Hamdani (may Allah have mercy on him) narrates: We were with Fadalah bin Ubayd (may Allah be pleased with him) at Rhodes in the land of the Romans when a companion of ours died. Fadalah (may Allah be pleased with him) gave orders about his grave and it was levelled. Then he said: I heard the Messenger of Allah (peace and blessings be upon him): he "used to order that they (graves) be levelled."
In a narration of Abu Salih (may Allah have mercy on him): Thumamah bin Shufayy (may Allah have mercy on him) says: We were with Fadalah bin Ubayd (may Allah be pleased with him) in the land of the Romans, and the rest of the hadith is the same as above.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6756
Abu al-Hayyaj al-Asadi says: Ali bin Abi Talib (may Allah be pleased with him) said to me: "I am sending you on the same mission on which the Messenger of Allah (peace and blessings be upon him) sent me: 'Do not leave any raised grave without levelling it with the ground, nor any image in a house without effacing it.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6757
Al-Qasim bin Muhammad narrates: I went to Sayyidah Aishah (may Allah be pleased with her) and said: O mother, uncover for me the grave of the Messenger of Allah (peace and blessings be upon him) and his two companions. So she uncovered for me three graves, neither raised high nor flat against the ground, spread with the red gravel of the open courtyard. I saw the Messenger of Allah (peace and blessings be upon him) at the front, and Abu Bakr (may Allah be pleased with him) with his head between the shoulders of the Messenger of Allah (peace and blessings be upon him), and Umar (may Allah be pleased with him) with his head at the feet of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6758
Abu al-Bada' says: I entered, with Mus'ab bin al-Zubayr (may Allah have mercy on him), the house in which the grave of the Prophet (peace and blessings be upon him) is. I saw their graves like flat, spread-out strips.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6759