A man came to the Prophet while he was delivering the sermon and asked him how to offer the night prayers. The Prophet replied, 'Pray two Rakat at a time and then two and then two and so on and if you are afraid of dawn (the approach of the time of the Fajr prayer) pray one Rak'a and that will be the wit'r for all the Rakat which you have prayed." 

Bukhari :: Book 1 :: Volume 8 :: Hadith 462