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."
حدثنا يزيد أخبرنا سليمان التيمي عن طاوس
عن النبي ﷺ قال صلاة الليل مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Its isnad is sahih, and it is a repetition of 4571.