What is the ruling on praying in a mosque that contains the shrine of a saint whose miracles are believed in, and is it permissible to pray behind an imam who believes in these beliefs?

It is forbidden to build mosques over graves, to bury the dead in a mosque, and to pray in a mosque that contains a grave. As for building a mosque near a grave, such that the grave is outside of it, this is not forbidden, and prayer in it is valid. Visiting a grave for admonition and reflection is permissible, but visiting it to supplicate to other than Allah is polytheism. The condition of the mosque's imam should be considered: if he believes that the saint fulfills needs, then one should not pray behind him. Otherwise, prayer behind him is valid.