Is the grave of prophet muhammed raised? Raising graves of awliya is permissible or not? Is there any hadith saying about the raising of graves?

"I[I]slam forbids erecting structures over graves, and commands that any such structures should be knocked down. But it is permitted to put a marker on the grave so that the family and friends of the deceased will know where it is. However, this marker should not be a structure or anything else that is not allowed in sharee'ah[/I]. "

Islam Question and Answer - Is it permissible to put a marker on a grave so that it will be known whose grave it is?
Please refer link for detailed answer.

Please take a look at the following links too.
Islam Question and Answer - Ruling on building mosques over graves





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