Sahih Muslim Book 4, Hadith Number 1828.

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

Sahl b. Abu Hathma reported that the Messenger of Allah (may peace be upon him) led his Companions in prayer in danger. He made them stand in two rows behind him. He led them who were close to him in one rak’ah. He then stood up and kept standing till those who were behind them observed one rak’ah. Then they (those standing in the second row) came in front and those who were in front went into the rear. He then led them In one rak’ah. He then sat down, till those who were behind him observed one rak’ah and then pronounced salutation.