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?

"Islam 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. "

Islam Question and Answer - Is it permissible to put a marker on a grave so that it will be known whose grave it is? (http://www.islamqa.com/en/ref/8991/raising%20marking%20graves)
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 (http://www.islamqa.com/en/ref/26312/Prophet%20Muhammad%20grave)





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