Quran 4:102

وَإِذَا كُنتَ فِيہِمۡ فَأَقَمۡتَ لَهُمُ ٱلصَّلَوٰةَ فَلۡتَقُمۡ طَآٮِٕفَةٌ۬ مِّنۡہُم مَّعَكَ وَلۡيَأۡخُذُوٓاْ أَسۡلِحَتَہُمۡ فَإِذَا سَجَدُواْ فَلۡيَكُونُواْ مِن وَرَآٮِٕڪُمۡ وَلۡتَأۡتِ طَآٮِٕفَةٌ أُخۡرَىٰ لَمۡ يُصَلُّواْ فَلۡيُصَلُّواْ مَعَكَ وَلۡيَأۡخُذُواْ حِذۡرَهُمۡ وَأَسۡلِحَتَہُمۡ‌ۗ وَدَّ ٱلَّذِينَ كَفَرُواْ لَوۡ تَغۡفُلُونَ عَنۡ أَسۡلِحَتِكُمۡ وَأَمۡتِعَتِكُمۡ فَيَمِيلُونَ عَلَيۡڪُم مَّيۡلَةً۬ وَٲحِدَةً۬‌ۚ وَلَا جُنَاحَ عَلَيۡڪُمۡ إِن كَانَ بِكُمۡ أَذً۬ى مِّن مَّطَرٍ أَوۡ كُنتُم مَّرۡضَىٰٓ أَن تَضَعُوٓاْ أَسۡلِحَتَكُمۡ‌ۖ وَخُذُواْ حِذۡرَكُمۡ‌ۗ إِنَّ ٱللَّهَ أَعَدَّ لِلۡكَـٰفِرِينَ عَذَابً۬ا مُّهِينً۬ا ١٠٢

When you are with them, and lead the Salat (Contact Prayer) for them, let some of you stand guard; let them hold their weapons, and let them stand behind you as you prostrate. Then, let the other group that has not prayed take their turn praying with you, while the others stand guard and hold their weapons. Those who disbelieved wish to see you neglect your weapons and your equipment, in order to attack you once and for all. You commit no error, if you are hampered by rain or injury, by putting down your weapons, so long as you remain alert. ALLAH has prepared for the disbelievers a shameful retribution.