"The Messenger of Allah forbade building over graves, making them larger or plastering over them." (One of two narrators) Sulaiman bin Musa added: "Or writing on them."
Sunan an-Nasa'i Book 21, Hadith 210
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Sunan an-Nasa'i Book 21, Hadith 209 Next Hadith → "The Messenger of Allah forbade plastering over graves, building over them or sitting on them."

Sunan an-Nasa'i Book 21, Hadith 211