Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.076.

Section : Allah explains how to pray during fear, safety or wartime.

“And when you are among them and arrange their prayers for them, let only a party of them stand with you (to worship) and let them take their arms. Then, when they have performed their prostrations, let them fall to the rear and let another party come to pray with you, and let them take their precautions and arms. They who disbelieve long for you to neglect your arms and your baggage, that they may attack. It is no sin for you to lay aside your arms, if rain impedes you or if you are sick. But take your precautions. Lo! Allah prepares for the disbelievers shameful punishment. When you have performed your prayer, remember Allah, standing, sitting and reclining. And when you are in safety, observe your prayer properly. Prayer at fixed hours has been enjoined on the believers” (an-Nisa’ 102-103).