From Sahih Bukhari (Hadith 993):
Narrated `Abdullah bin `Umar:The Prophet (ﷺ) said, "Night prayer is offered as two rak`at followed by two rak`at and so on, and if you want to finish it, pray only one rak`a which will be witr for all the previous rak`at." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-rak`at prayer as witr and all that is permissible. I hope there will be no harm in it
Reference: Bukhari 993