It wasnarrated that Ibn 'Umar said:
"The Messenger of Allah (ﷺ)usedto pray (voluntary prayers) at night two by two, and he wouldprayone Rak'ah of Witr." (RU)
English reference (RU): Vol. 1, Book 5, Hadith 1174 (RU)
Arabic reference (RU): Book 5, Hadith 1230 (RU)