Mishkat al-Masabih 5 Funerals كتاب الجنائز 170

وَعَنْ جَابِرٍ قَالَ: نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ يُجَصَّصَ الْقَبْرُ وَأَنْ يُبْنَى عَلَيْهِ وَأَنْ يُقْعَدَ عَلَيْهِ. رَوَاهُ مُسْلِمٌ

Jabir said God’s messenger forbade that graves should be plastered with gypsum, that any erection should be built over them, and that people should sit on them. Muslim transmitted it.