What is the ruling on praying in a mosque built over the grave of a righteous saint, knowing that the grave does not contain a body, and that the actual body is submerged under the waters of a river near the mosque?

Prayer is not valid in mosques built over graves, and building mosques over them is one of the major sins. This ruling also applies to mosques built over imaginary graves, because polytheistic and idolatrous practices occur at revered graves, whether real or not. The reason for prohibiting the building of mosques over graves is to block the means leading to extremism that results in polytheism, and this applies to imaginary graves. Scholars have differentiated between visible graves, where polytheistic acts occur, and obliterated graves, which do not entail such consequences. Therefore, it is not permissible to build mosques over imaginary graves, nor is prayer valid therein.