Sahih Bukhari

Narated By 'Abdullah bin 'Umar  : The Prophet said, "Night prayer is offered as two Rakat followed by two Rakat and so on, and if you want to finish it, pray only one Raka which will be Witr for all the previous Rakat." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-Rakat prayer as Witr and all that is permissible. I hope there will be no harm in it."