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 عمرو بن عثمان ومحمد بن صدقة قالا حدثنا محمد بن حرب عن الزبيدي عن الزهري
عن النبي ﷺ قال صلاة الليل مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Bukhari, no. 1137, and Muslim, no. 146/749, transmitted it from the hadith of Zuhri through this route (see the following hadith, no. 1673). It is also in Kubra, no. 473.