Ibn Umar, when asked about the prayer of fear, would describe its manner. Then he said, "If the fear is greater than that, then pray on foot, standing on your feet, or mounted, whether facing the qiblah or not."
أن ابن عمر كان إذا سئل عن صلاة الخوف قال: فذكر صفة صلاة الخوف ثم قال: فإن كان خوف هو أشد من ذلك صلوا رجالا قياما على أقدامكم، أو ركبانا مستقبلي القبلة أو غير مستقبليها.
Malik narrated it in "Prayer of Fear" (3) from Nafi, as mentioned. Malik said, Nafi said, "I do not think that Abdullah bin Umar mentioned that except from the Messenger of Allah ﷺ."Bukhari narrated it in tafsir (4535) through the route of Malik, with the same wording. Muslim narrated it in "Prayer of Fear" (839:306) from the hadith of Musa bin Uqba, from Nafi. In it, it states, Ibn Umar said, "If the fear is greater than that, then pray mounted or standing, gesturing with a nod."His statement, "on foot" [Baqarah 2:239], means pray while walking, which is the plural of "walker," like "standing" and "standers."And his statement, "or mounted" [Baqarah 2:239], means or pray while mounted, the plural of "rider."