Sahih Bukhari, Chapter: Prayers (Salat)

Ibn 'Umar said, "While the Prophet was on the pulpit, a man asked him how to offer the night prayers. He replied, 'Pray two Rakat at a time and then two and then two and so on, and if you are afraid of the dawn (the approach of the time of the Fajr prayer) pray one Rak'a and that will be the witr for all the Rakat which you have offered." Ibn 'Umar said, "The last Rakat of the night prayer should be odd for the Prophet ordered it to be so.