Ruling on building a mosque in a graveyard

It is not permissible for him to do that, and it should
be knocked down. (Words of Imaam al-Nawawi).

Taking graves as mosques or places of worship is one of the customs of
the Jews with whom Allaah is angry. Abu Hurayrah (may Allaah be pleased with him) reported
that the Messenger of Allaah
(peace and blessings of Allaah be upon him) said: May
Allaah destroy the Jews! They have taken their graves as places of worship.

(Agreed
upon). Ibn Abbaas (may Allaah be pleased with him) said: The
Messenger of Allaah
(peace and blessings of Allaah be upon him) cursed the women who
visit graves and those who build mosques over them and place lamps on them. Some of
them claim that this was because at that time they were still close to the time when
people had worshipped idols, but Ibn Daqeeq al-Eed refuted this argument. This
indicates that it is haraam to place lamps on graves because this could lead to false
beliefs.

In addition to the issue of building mosques over graves, this person’s
actions also involve going against the conditions set by the one who endowed this land.
Moreover, he is denying the Muslim dead space in which to be buried. Adding any extra
structure over a grave as the ignorant people among the Muslims do is haraam, as stated by
the fuqaha and ulama, let alone building mosques over graves. So the
imaam (Imaam al-Nawawi) ruled that whatever mosques had been built over the graves should
be torn down.

(Comment in the margin of Fataawa al-Imaam al-Nawawi, p. 65).