Is it permissible to pray the funeral prayer in the mosque?

The Sunnah is to pray over the deceased in a designated place for funerals outside the mosque, as the Prophet, peace and blessings be upon him, did when he prayed over Najashi in the prayer area.

However, there is no harm in praying over the deceased inside the mosque. Aisha, may Allah be pleased with her, ordered that the funeral of Sa'd ibn Abi Waqqas be brought through the mosque so she could pray over him. She also said that the Prophet, peace and blessings be upon him, prayed over Suhail ibn al-Bayda' in the mosque.

Ibn Qudamah said: "There is no harm in praying over the deceased in the mosque, provided there is no fear of soiling it." Sheikh Ibn Uthaymeen also issued a fatwa permitting prayer over the deceased in the mosque.