What is the ruling on collecting the remains of martyrs?

The remains of martyrs are buried like any other deceased person, each martyr in a grave designated for them. However, if this becomes difficult due to a large number of deaths from an epidemic or war, then it is permissible to bury two or three individuals in one grave, with the most virtuous among them placed closest to the Qiblah, as the Prophet, peace and blessings be upon him, did with the martyrs of Uhud.