Sahih Muslim Book 4, Hadith Number 1824.

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

Salim b. Abdullah b. ‘Umar reported: The Messenger of Allah (may peace he upon him) led one of the two groups In one rak’ah of prayer in danger, while the other group faced the enemy. Then they (the members of the first group) went back and replaced their companions who were facing the enemy and then they (the members of the second group) came and the Apostle of Allah (may peace be upon him) led them in one rak’ah of prayer. Then the Apostle of Allah (may peace be upon him) pronounced salutation, and then they (the members of the Ant group) completed the rak’ah and they (the members of the second group) completed the rak’ah. This Hadith has been narrated by another chain of transmitters.