Jabir narrates that the Messenger of Allah (peace and blessings of Allah be to him) has forbidden (the believers from) making the grave firm with plaster, from sitting on it, and from erecting a structure over it.
This hadīth cuts at the very roots of grave worship, for this practice can be indulged into only where graves are made firm and concrete, and where structures are built over them. Islam does not allow the graves of saints and the revered to be turned into dargahs or mausoleums. The magnificent mausoleums built on the graves of saints and sages are absolutely contrary to the spirit of Islam. Islam wants graves to be extremely simple so that they may remind the visitors of the transitoriness of life on earth, their ultimate return to Allah, help them stick to Oneness of Allah, and above all keep them from indulgence in grave worship. But the magnificent edifices over graves-turned-dargahs appear so attractive and splendid that they have become common hunts of people. The respect paid to them touches the limits of worship, and the holiness bestowed upon them has made them appear as divine to such an extent that even sajda (prostration) is offered at these dargahs, prayers and pledges are made there, offerings are placed at the shrines, and benefits are said to be derived therefrom. These graves are decorated with silken covers and flowery sheets. And the irony is that this grave-worshipping section of Muslims does not think even a bit that this action of theirs affects their belief in tawheed.