The Messenger of Allah ﷺ forbade building over a grave or plastering it with gypsum.
حدثنا حسن حدثنا ابن لهيعة حدثنا يزيد بن أبي حبيب عن ناعم مولى أم سلمة
نهى رسول الله ﷺ أن يبنى على القبر أو يجصص
Its chain of transmission is hasan, on account of Ibn Lahia. Naim is Ibn Ajayl, the freed slave of Umm Salama, and he is trustworthy, his hadith being found in Muslim. Haythami likewise graded the hadith hasan (3/61), and it has been mentioned many times before.