The Messenger of Allah ﷺ forbade plastering graves with gypsum, sitting on them, or building structures over them.
نهى رسول الله ﷺ أن يجصص القبر، وأن يقعد عليه، وأن يبنى عليه.
Muslim narrated it in "The Funerals" (970) through several routes from Ibn Jurayj, who said, "Abu Zubayr informed me that he heard Jabir bin Abdullah," and he mentioned the hadith in the same way.Ayub narrated it from Abu Zubayr, from Jabir, who said, "He forbade the plastering of graves."Abu Dawud added (3226), through the route of Sulayman bin Musa, from Jabir, "and that anything be written on it."Ibn Majah also narrated it (1563) through the route of Hafs bin Ghiyath, from Ibn Jurayj, from Sulayman bin Musa, from Jabir, and he mentioned the hadith with the wording, "The Messenger of Allah ﷺ forbade that anything be written on a grave."Sulayman bin Musa never met Jabir.Hakim narrated it in Mustadrak (1/370) through the route of Hafs bin Ghiyath Nakhai, "Ibn Jurayj told us, from Abu Zubayr, from Jabir," and in it he combined all the wordings of the hadith, namely, "The Messenger of Allah ﷺ forbade that a grave be built upon, or plastered, or sat upon, and he forbade that anything be written on it."He said, "It meets the standard of Muslim, and he narrated with its chain everything except the writing," for it is a solitary but sahih wording, "and likewise Abu Muawiya narrated it from Ibn Jurayj." End of quote.Then he narrated it through his own route and said, "These chains are sahih, but practice does not follow them, for the imams of the Muslims from East to West have writing on their graves, and this is a practice that later generations adopted from earlier generations." End of quote.Dhahabi rejected this, saying, "You have said nothing of substance, and we do not know of any companion who did that, rather it is something that some of the followers and those after them introduced, and the prohibition never reached them."Tajsis and taqsis both mean building with gypsum plaster, which is also called qass or qassa, and jassas and qassas are the same thing. When the gypsum is mixed with ash, it is called jiyar.See Mufhim (2/626).On this subject there is also a report from Umm Salama, who said, "The Messenger of Allah ﷺ forbade that a grave be built upon or plastered."Imam Ahmad narrated it (26555, 26556) through two routes. The first, from Hasan, "Ibn Lahia told us, Yazid bin Abu Habib told us, from Naim, the freed slave of Umm Salama," and he mentioned it.The second, from Ali bin Ishaq, "Abdullah told us, Ibn Lahia informed us, Yazid bin Abu Habib told me, from Naim, the freed slave of Umm Salama, that the Prophet ﷺ forbade that a grave be plastered, or built upon, or sat upon." He, meaning Abdullah bin Ahmad, said, "My father said, Umm Salama is not mentioned in it." End of quote.The second route is the sahih one, because Abdullah, who is Ibn Mubarak, is one of the Abadila who heard from Ibn Lahia before his books were burned, except that it is a mursal report, as Abdullah bin Ahmad related from his father.