(1) باب مَا جَاءَ فِي الْوِتْرِ
Narrated Ibn `Umar:
Once a person asked Allah's Messenger (ﷺ) (ﷺ) about the night prayer. Allah's Apostle (ﷺ) replied, "The night prayer is offered as two Rak`at followed bytwo Rak`at and so on and if anyone is afraid of the approaching dawn (Fajr prayer) he should pray one Rak`ah and this will be a Witr for all the Rak`at which he has prayed before."
Reference: Sahih al-Bukhari 990
In-book reference: Book 14, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 16, Hadith 105