Witr is an optional Muslim prayer yet that can be said at night between Isha'a (night) and Fajr (morning) prayers. The Prophet (Muhammed) said, according to Ibn Umar, night prayer is offered as two rakat followed by two rakat.
References
- Translation http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/bukhari/016.sbt.html of Sahih Bukhari, Book 16: Witr Prayer