Allah prescribed the prayer through the tongue of your Prophet as four units while resident, two units while traveling, and one unit during times of fear.
فرض الله الصلاة على لسان نبيكم في الحضر أربعا، وفي السفر ركعتين، وفي الخوف ركعة.
Muslim narrated it in "The Prayer of Travelers" (687) through several routes from Mujahid and others, from Ibn Abbas, as mentioned above. Some scholars held this view regarding severe fear, among them Ataa, Tawus, Hasan, Mujahid, Hakam, Hammad, Qatada and others, all of whom said, "In severe fear, one rakat suffices, performed with a gesture." Ishaq bin Rahawayh said, "In a state of severe danger, one rakat suffices, performed with a gesture. If one cannot manage that, then one sajdah. If one cannot manage even that, then a single utterance of takbir, since it is a remembrance of Allah."As for the rest of the scholars, they said that the prayer during severe fear should not be reduced in the number of rakats, but rather one should pray two rakats as best as possible, facing whatever direction one can, whether standing or riding, making gestures in place of the normal movements. This is the view held by the people of legal reasoning, Malik, and Shafii. Imam Ahmad said, "Every hadith narrated concerning the chapters on the prayer of fear may be acted upon." He also said, "There are six or seven ways narrated regarding it, and all of them are permissible." See Maalim of Khattabi.What Imam Ahmad held is among the most balanced positions, avoiding the need for either forced reinterpretation or weakening of the reports, as will be explained shortly from the words of Bayhaqi, may Allah have mercy on him.