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 ﷺ أن يجصص القبر، وأن يقعد عليه، وأن يبنى عليه. ((رواه مسلم))