The Book of Prayer - Funerals
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حَفْصُ بْنُ غِيَاثٍ، عَنِ ابْنِ جُرَيْجٍ، عَنْ أَبِي، الزُّبَيْرِ عَنْ جَابِرٍ، قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُجَصَّصَ الْقَبْرُ وَأَنْ يُقْعَدَ عَلَيْهِ وَأَنْ يُبْنَى عَلَيْهِ .Jabir said:Allah's Messenger (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
From Sahih Muslim, whose reports the scholars accept as authentic.