It is forbidden to build mosques on graves

 256. ´Aishah narrated: Umm Habibah and Umm Salamah made a mention before the Messenger of Allah salla alayhi wa sallam of a church they saw in Abyssinia, in which there were pictures. The Messenger of Allah salla alayhi wa sallam said: ´´When a pious person among those people dies, they build a place of worship on his grave, and then decorate it with such pictures. They are the most evil of creatures in the Sight of Allah, on the Day og Resurrection.´´