Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) and Sulayman bin Musa narrate that the Messenger of Allah (peace and blessings be upon him) forbade that a structure be built over the grave, or that extra earth be added to it, or that it be plastered.
Also, Jabir (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he said: "No extra earth should be added to his grave."
The first hadith suffices; Aban is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6735
It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that a lahd (niche grave) was dug for the Prophet (peace and blessings be upon him) and unbaked bricks were set up over him. Then he mentioned the rest of the hadith and said that the grave of the Prophet (peace and blessings be upon him) was raised from the ground by about a hand-span. This is how I found it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6736
Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father that water was sprinkled on the grave of the Prophet (peace and blessings be upon him), pebbles were placed on it, and the grave was raised to the height of a hand-span.
This is mursal (the Companion is missing from the chain). Al-Waqidi narrated it with a chain of his own from Jabir, and that is rejected.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6737
Thumamah bin Shufayy says: We set out on a campaign in the time of Mu'awiyah (may Allah be pleased with him) towards these lands, and Fadalah bin Ubayd (may Allah be pleased with him) was in command over us. A cousin of mine, called Nafi' bin Abd, died. He says: Fadalah (may Allah be pleased with him) stood at his grave, and when we had buried him he said: Lighten the soil over him, for the Messenger of Allah (peace and blessings be upon him) "used to order us to level the graves."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6738