Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.069.

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)