The Messenger of Allah ﷺ prohibited building on a grave or adding to it or leveling or plastering it.

This was narrated by Abān b. Abū Ayyāsh from al-Ḥasan and by Abū Naḍrah from Jabir from the Prophet ﷺ who said, "And do not add earth on top of his grave."  

أَنَّ رَسُولَ اللهِ ﷺ نَهَى أَنْ يُبْنَى عَلَى الْقَبْرِ أَوْ يُزَادَ عَلَيْهِ أَوْ يُجَصَّصَ وَرَوَاهُ أَبَانُ بْنُ أَبِي عَيَّاشٍ عَنِ الْحَسَنِ وَأَبِي نَضْرَةَ عَنْ جَابِرٍ عَنِ النَّبِيِّ ﷺ قَالَ وَلَا يُزَادُ عَلَى حُفَيْرَتِهِ التُّرَابُ  

[Machine] The Prophet ﷺ was cursed and milk was poured over him as a form of insult. It was mentioned in the hadith that he said, "His grave was raised from the ground to a height of a hand span as I witnessed it."  

أَنَّ النَّبِيَّ ﷺ أُلْحِدَ لَهُ لَحْدًا وَنُصِبَ عَلَيْهِ اللَّبِنَ نَصْبًا وَذَكَرَ الْحَدِيثَ قَالَ وَرُفِعَ قَبْرُهُ مِنَ الْأَرْضِ نَحْوًا مِنْ شِبْرٍ كَذَا وَجَدْتُهُ  

[Machine] That the Prophet ﷺ was sprinkled with water on his grave, and pebbles from the pebbles of Al-Arshah were placed on it, and his grave was raised to the length of a handspan. This is a hadith reported by Al-Waqidi with a chain of narrators from Jabir. And this is refuted.  

أَنَّ النَّبِيَّ ﷺ رُشَّ عَلَى قَبْرِهِ الْمَاءُ وَوُضِعَ عَلَيْهِ حَصْبَاءُ مِنْ حَصْبَاءِ الْعَرْصَةِ وَرُفِعَ قَبْرُهُ قَدْرَ شِبْرٍ وَهَذَا مُرْسَلٌ وَرَوَاهُ الْوَاقِدِيُّ بِإِسْنَادٍ لَهُ عَنْ جَابِرٍ وَذَلِكَ يُرَدُّ  

[Machine] Lighten the soil from him, for the Messenger of Allah ﷺ used to instruct us to level the graves.  

خَفِّفُوا عَنْهُ التُّرَابَ؛ فَإِنَّ رَسُولَ اللهِ ﷺ كَانَ يَأْمُرُنَا بِتَسْوِيَةِ الْقُبُورِ