It is narrated from Hazrat Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) forbade building solid structures over graves or applying plaster to them.
Hadith Referenceمسند احمد / مسند النساء / 26555
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد اختلف فيه على ابن لهيعة