Sahih Muslim Book 4, Hadith Number 1825.

Chapter : Excellence of observing two Rak’ahs before the evening prayer.

Ibn Umar reported that the Messenger of Allah (may peace be upon him) observed prayer in danger on some day (in this way): a group stood with him (the Holy Prophet) (for prayer) and the other group stood In front of the enemy. Then those who were with (him) observed one rak’ah of prayer and they went back and the others came and they observed one rak’ah (with him). Then both the groups completed one rak’ah each. Ibn Umar said: When there is greater danger, then observe prayer even on the ride or with the help of gestures in a standing posture.