(1) باب مَا جَاءَ فِي الْوِتْرِ
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: Sahih al-Bukhari 993
In-book reference: Book 14, Hadith 4
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 16, Hadith 107