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