What is the ruling on praying in a mosque that contains a grave, and is it true that the three Imams (Abu Hanifa, Malik, and Shafi'i) deemed it permissible and not disliked unless the grave is in front of the worshiper, and that it was Ahmad ibn Hanbal who prohibited the prayer and ruled it invalid? And is the disagreement specific to cases where the grave is inside the mosque and not separated from it?

It is unanimously forbidden to take graves as mosques, due to the Prophet's (peace be upon him) prohibition of that. He said: "Indeed, those who were before you used to take graves as mosques. Do not take graves as mosques, for I forbid you from that." The four Imams agreed on the impermissibility of building mosques over graves, and the impermissibility of praying towards a grave or on it. If the mosque was built after the grave, then either the mosque is removed or the image of the grave is removed. No obligatory or supererogatory prayer is to be performed in a mosque that is over a grave.