The Prophet ﷺ said, "The night prayer is to be offered two units at a time. When you fear the approach of dawn, pray a single unit as witr."
أخبرنا قتيبة قال حدثنا الليث عن نافع
عن النبي ﷺ قال صلاة الليل مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Bukhari transmitted it, in "Prayer", chapter "Sitting in a circle and sitting in the masjid", h: 990, 472, and Muslim, in "Prayer of the Traveler", chapter "Night prayer being two by two", h: 150/751, from the hadith of Nafi through him. Tirmidhi also transmitted it, h: 437, from Qutayba through him, and said, "hasan sahih."