Sahih Bukhari: Hadith 465

Ibn `Umar said, "While the Prophet (ﷺ) was on the pulpit, a man asked him how to offer the night prayers. He replied, 'Pray two rak`at 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 rak`at which you have offered." Ibn `Umar said, "The last rak`at of the night prayer should be odd, for the Prophet (ﷺ) ordered it to be so.