It is narrated from Hazrat Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) forbade building a solid structure over a grave, applying plaster to it, or sitting on it.
Hadith Referenceمسند احمد / مسند النساء / 26556
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد اختلف فيه على ابن لهيعة