Mukhtasar Bukhari (Abridged) 525 — Prayer of fear can be performed

In another narration, Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) reports (the statement of the Messenger of Allah, peace and blessings be upon him): "If the enemy are more than them (the Muslim soldiers), then (these Muslim soldiers) should perform the prayer, whether on foot or mounted, in whatever way is possible."