Sahih Bukhari Volume 005, Book 059, Hadith Number 456.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 59. Military Expeditions Led by the Prophet (PBUH) (Al-Maghaazi)
Narrated By 'Abdullah bin 'Umar : Allah's Apostle led the Fear-prayer with one of the two batches of the army while the other (batch) faced the enemy. Then the first batch went away and took places of their companions (i.e. second batch) and the second batch came and he led his second Rak'a with them. Then he (i.e. the Prophet: finished his prayer with Taslim and then each of the two batches got up and completed their remaining one Rak'a.