What is the ruling on praying in a mosque built upon a very old graveyard, knowing that no other place is available, and the other mosque is 3 km away, and the congregational prayer may be missed if we do not pray in it?

Prayer is not permissible in mosques built upon graves or in which graves have been incorporated. There are numerous, consecutively transmitted texts from the Prophet, peace and blessings be upon him, that forbid this. A mosque and a grave do not coexist in Islam; whichever of them was introduced later must be removed. The imams have agreed that a mosque should not be built upon a grave, nor is it permissible to bury a deceased person in a mosque. If a mosque is built upon a grave, then either the mosque must be removed or the image of the grave must be removed. Neither obligatory nor supererogatory prayers are to be performed in a mosque built upon a grave, as it is forbidden. If possible, pray in a mosque that has no graves within it; otherwise, work towards removing the images of the graves.