I heard the Prophet (ﷺ) forbid to sit on the grave, to plaster it with gypsum, and to build any structure over it.
Sunan Abi Dawud Book 21, Hadith 137
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Sunan Abi Dawud Book 21, Hadith 136 Next Hadith → 'Uthman said: "or anything added to it." Sulaiman b. Musa said: "or anything written on it." Musaddad did not mention in his version the words "or anything added to it."

Abu Dawud said: The word "and that" (wa an) remained hidden to me.

Sunan Abi Dawud Book 21, Hadith 138