Hadith 1767
Jabir (May Allah be pleased with him) said:
عن جابر رضي الله عنه قال: نهى رسول الله صلى الله عليه وسلم أن يجصص القبر، وأن يقعد عليه، وأن يبنى عليه. ((رواه مسلم))
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].
In plain words
The Prophet forbade plastering graves into permanent structures, sitting on them, or building over them. This keeps burial simple and devotion turned to Allah alone.
Reference : Riyad as-Salihin 1767 In-book reference : Book 17, Hadith 257