The Messenger of Allah ﷺ was asked about the night prayer. He said, "Two by two, and when you fear the approach of dawn, pray a single rakah as witr."
حدثنا أبو المغيرة حدثنا عبد الله بن العلاء يعني ابن زبر حدثني سالم بن عبد الله
سئل رسول الله ﷺ كيف صلاة الليل فقال مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Its isnad is sahih. Abdullah bin Ala bin Zabr, with a fatha on the zay and no vowel on the ba, the Damascene, is trustworthy. Ibn Main and Abu Dawud and others considered him trustworthy, and likewise Ibn Saad considered him trustworthy in al-Tabaqat 7/2/171. The hadith is an abridgment of 6008.