Jabir relates that the Messenger of Allah ﷺ has forbidden making the grave firm with plaster, from sitting on it and from erecting a structure over them.
(Muslim)
This hadīth cuts at the very roots of grave worship, for this practice can be indulged into only when graves are made firm and pucca, and when structures are built over them to make dargahs (shrines). Islam does not allow graves of sages to be turned into dargahs or mausoleums. The magnificent mausoleums which the Muslims have built on the graves of their saints and sages are absolutely contrary to the spirit of Islam. Islam wants to see the graves in an extremely simple form, so that the people remain attached to the belief in Oneness of Allah, and they may not indulge in grave worship. But the magnificent edifices over graves are made so attractive and splendid that they have become common hunts of people. The respect paid to them borders at worshipping, and the holiness bestowed upon them has made them appear as divine to such an extent that even sajdah (prostration) is performed at these dargahs, prayers and pledges are made there, offerings are placed at shrines, and benefits are said to be derived therefrom. The irony is that the grave-worshipping Muslims think this action of theirs does not affect their belief in tawheed. While, according to the teachings of the Qur’ān and Sunnah, this covering of the grave is tantamount to shroud of the religion, and building of splendid mausoleums amounts to digging the grave of Islam.