Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) says: I heard the Prophet (peace and blessings be upon him): he forbade that a man should sit on a grave, or that it be plastered, or that a structure be built over it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6762