A man from the Muslims asked the Messenger of Allah ﷺ how the night prayer should be performed. He replied, "The night prayer is two units at a time. When you fear the approach of dawn, pray a single unit as witr."
أخبرنا أحمد بن محمد بن المغيرة قال حدثنا عثمان عن شعيب عن الزهري عن سالم
سأل رجل من المسلمين رسول الله ﷺ كيف صلاة الليل فقال صلاة الليل مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Bukhari transmitted it in Tahajjud, chapter "How was the prayer of the Prophet?" etc., no. 1137, from the hadith of Shuayb bin Abu Hamza, and Muslim, no. 146/749 (see the previous hadith), from the hadith of Zuhri, with the same wording.