It is haraam to bury the dead inside mosques

The Prophet (blessings and peace of Allah be upon him)
forbade burying people in the mosque and building mosque over graves; he
also cursed those who do that. That was when he was on his deathbed and
warned his ummah. He (blessings and peace of Allah be upon him) stated
that this was the action of the Jews and Christians and it was also a
means that leads to associating others with Allah, because building mosque
over graves and burying the dead inside mosques leads to associating the
occupants of these graves in worship with Allah, may He be glorified and
exalted, because people may come to believe that the occupants of these
graves who are buried inside the mosque can bring benefit or cause harm,
or that they have some special status which mean that people should seek
to draw close to them by doing acts of worship and obedience to them
instead of Allah, many He be glorified and exalted. 

The Muslims must beware of this dangerous phenomenon and
ensure that the mosques are free of graves and are established on the basis
of Tawheed and sound belief. Allah says (interpretation of the meaning):
“And the mosques are for Allaah (Alone), so invoke not anyone along with
Allaah” [al-Jinn 72:18]. 

The mosque should be for Allah alone, may He be glorified and
exalted, and free of any manifestations of shirk, places where Allah alone
is worshipped with no partner or associate. This is what is required of the
Muslims. And Allah is the source of strength. End quote. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on
him). 

Fataawa al-‘Aqeedah, p. 26. 

For more information please see the answer to question number
120214 

And Allah knows best.