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]. (ES)
App reference (ES): Book 18, Hadith 257 (ES)
Arabic/English book reference (ES): Book 18, Hadith 1767 (ES)