Riyad as-Salihin 1767
Jabir (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) forbade that the graves should be plastered (made into permanent structures), used as sitting places (for the people) or building over them. [Muslim] .
عن جابر رضي الله عنه قال‏:‏ نهى رسول الله صلى الله عليه وسلم أن يجصص القبر، وأن يقعد عليه، وأن يبنى عليه‏.‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏