Messenger of Allah ﷺ forbade that the graves should be plastered or they be used as sitting places (for the people), or a building should be built over them.  

نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يُجَصَّصَ الْقَبْرُ وَأَنْ يُقْعَدَ عَلَيْهِ وَأَنْ يُبْنَى عَلَيْهِ