Surah An-Nisa'...

"And when you are among them (Muslim forces) and keep up the prayer for them, then let a party of them stand up (in prayer) with you, carrying their arms.

Then when they have performed (their) prostrations, let them (continue the second unit solitarily and) take their precaution in your rear, and let another party who have not prayed come and pray with you (from the second unit), and let them take their precautions and their arms; (for) those who disbelieve desire that you should be heedless of your weapons and your baggage, then they would attack you all at once.

And it is no sin for you to lay down your arms if you are annoyed with rain or if you are sick, but take your precautions. Verily Allah has prepared a disgraceful chastisement for the infidels." In the sixth year AH the holy Prophet (S), accompanied some believers, set out towards Mecca.

On the way, when they reached the region of Hudaybiyyah, they encountered with Khalid-ibn-Walid supported by two hundred men who had sheltered thereabout and were waiting for hindering the Prophet (S) from arriving in Mecca when Bilal said the prayer call and the prayer was established. Khalid-ibn-Walid devised to profit by the time when all Muslims were busy establishing the evening prayer and to attack them at a single instance. The verse was revealed and informed the Prophet (S) of the plot.

So, seeing that miracle appeared through divine revelation, Khalid embraced Islam. In this prayer, after performing the first unit of prayer, the first group stand up and complete the rest of the prayer, but the prayer leader waits a little so that the other group may come and join the second unit of the prayer carrying their weapons. "And when you are among them (Muslim forces) and keep up the prayer for them, then let a party of them stand up (in prayer) with you, carrying their arms.