Covering graves or tombs is not permissible, because this involves unnecessary expense for an invalid cause, and may mislead the common man. ‘Aishah reported: “The Prophet, peace be upon him, left on a raid. I covered the door with a piece of cloth. The Prophet, peace be upon him, noticed the cloth when he returned. He pulled at it until he tore it. Then he said: ‘Allah did not order us to cover stones and clay’.” (Reported by Bukhari and Muslim)
Section : The Prohibition against Covering Graves.