Ibn 'Umar (May Allah be pleased with them) reported:
The Prophet (ﷺ) said, "Salat during the night should consist of pairs of but if you fear that morning is near, then pray one Rak'ah as Witr."[Al-Bukhari and Muslim]. (RU)
App reference (RU): Book 9, Hadith 178 (RU)
Arabic/English book reference (RU): Book 9, Hadith 1168 (RU)