وعنه قال: كان النبي صلى الله عليه وسلم، يصلي من الليل مثنى مثنى، ويوتر بركعة
The Prophet (sallallahu alayhi wa sallam) performed the night prayer in pairs (i.e., Rakah) and made it odd number by observing one Rakah (as Witr).
Riyad as-Salihin 1169, Book 9, The Book of Virtues, Hadith 179. 1 min read