Allah Almighty says, «‏‏‏‏فَإِنْ خِفْتُمْ فَرِجَالًا أَوْ رُكْبَانًا» ‏‏‏‏ "In times of fear, pray while walking on foot or riding."

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when he was asked about the prayer of fear (Salat al-Khawf), he said: The imam and a group of people will stand, and the imam will lead them in one rak'ah, while the other group who have not prayed will stand between the imam and the enemy. Then, when the group with the imam has prayed one rak'ah with him, without saying salam, they will move to the place of those who have not prayed, and those who have not prayed will come forward and perform one rak'ah with the imam. Then the imam will say salam, and he will have completed two rak'ahs, then both groups will each complete their remaining one rak'ah (and say salam). If the fear is even more severe than this, then they should pray while walking, standing, or riding, facing the qiblah or not facing the qiblah. Nafi' says: I think that Sayyiduna Ibn Umar (may Allah be pleased with him) narrated this (method and form of prayer) from the Messenger of Allah (peace and blessings be upon him) himself.

Imam Sahib narrates from his teacher, Muhammad bin Yahya, with his chain of transmission that Nafi' said: Indeed, Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates this manner and form exactly from the Messenger of Allah (peace and blessings be upon him).