The Quran

102

وَإِذا كُنتَ فيهِم فَأَقَمتَ لَهُمُ الصَّلاةَ فَلتَقُم طائِفَةٌ مِنهُم مَعَكَ وَليَأخُذوا أَسلِحَتَهُم فَإِذا سَجَدوا فَليَكونوا مِن وَرائِكُم وَلتَأتِ طائِفَةٌ أُخرىٰ لَم يُصَلّوا فَليُصَلّوا مَعَكَ وَليَأخُذوا حِذرَهُم وَأَسلِحَتَهُم ۗ وَدَّ الَّذينَ كَفَروا لَو تَغفُلونَ عَن أَسلِحَتِكُم وَأَمتِعَتِكُم فَيَميلونَ عَلَيكُم مَيلَةً واحِدَةً ۚ وَلا جُناحَ عَلَيكُم إِن كانَ بِكُم أَذًى مِن مَطَرٍ أَو كُنتُم مَرضىٰ أَن تَضَعوا أَسلِحَتَكُم ۖ وَخُذوا حِذرَكُم ۗ إِنَّ اللَّهَ أَعَدَّ لِلكافِرينَ عَذابًا مُهينًا

Muhammad Asad

Thus, when thou art among the believers¹ and about to lead them in prayer, let [only] part of them stand up with thee, retaining their arms. Then, after they have finished their prayer, let them provide you cover² while another group, who have not yet prayed, shall come forward and pray with thee, being fully prepared against danger and retaining their arms: [for] those who are bent on denying the truth would love to see you oblivious of your arms and your equipment, so that they might fall upon you in a surprise attack.³ But it shall not be wrong for you to lay down your arms [while you pray] if you are troubled by rain⁴ or if you are ill; but [always] be fully prepared against danger. Verily, God has readied shameful suffering for all who deny the truth!

Lit., "among them". The "thou" in this sentence refers, primarily, to the Prophet and, by implication, to the leader of every group of believers at war with "those who deny the truth".
Lit., "when they have prostrated themselves, let them [i.e., the other group] be behind you". This idiomatic expression is not to be taken literally: in classical Arabic usage, the phrase kana min wara ika (lit., "he was behind thee") signifies "he protected thee" or (in military parlance) "he covered thee", and is not meant to describe the physical relative position of the two persons or groups.
Lit., "turn upon you in one turning".
I.e., if there is a risk of their weapons being damaged by exposure to unfavourable weather conditions, the warriors are exempted from the obligation of keeping their arms with them while praying. This exemption applies, of course, only to such of the soldiers as are in charge of particularly sensitive weapons; and the same applies to the individual cases of illness mentioned in the sequence. It must, however, be remembered that the term matar (lit., "rain") is often used in the Qur'an to denote "an affliction": and if we adopt this meaning, the above phrase could be rendered as "if you suffer from an affliction"- thus allowing for a wide range of possible emergencies.