618 The congregational prayer in danger in face of the enemy rests on the principle that the congregation should be divided into two parties; one party prays while the other watches the enemy, and then the second party comes up to prayers while the first falls back to face the enemy; either party does only one or two Rak'ahs, or about half the congregational prayer; every precaution is taken to prevent a rush by the enemy; even while at prayers armour and arms need not be put off except when rain is likely to cause inconvenience to the wearer and damage to the arms, or when illness or fatigue causes the wearer's strength to fail. Details can he varied according to circumstances, as was actually done by the Prophet at different times.