4:102An-Nisaa

When you O Muhammad (s) are present among them while you all fear an enemy and you stand to lead them in prayer this type of address is customary in the Qur’ān let a party of them stand with you while another party stand back and let them the party standing with you take their weapons with them. Then when they have performed their prostrations that is when they have prayed let them the other party be behind you on guard until you complete the prayers; thereupon let this party go on guard and let another party who have not prayed come and pray with you taking their precautions and their weapons with them until you have completed the prayers. The Prophet s did this once at Batn Nakhla as reported by the two Shaykhs Bukhārī and Muslim. The disbelievers wish when you have stood up to pray that you should be heedless of your weapons and your baggage that they may descend upon you all at once by making an assault against you and capturing you and herein is the reasoning behind keeping weapons on oneself. You are not at fault if rain bothers you or if you are sick to lay aside your weapons and not carry them this implies that when there is no such excuse it is compulsory to carry them and this is one of two opinions held by al-Shāfi‘ī on this matter; the other opinion is that this precaution constitutes a sunna and this is the more preferable opinion. But take your precautions against the enemy and be on your guard as best you can; God has prepared for the disbelievers a humiliating chastisement.