A man asked the Messenger of Allah ﷺ about the night prayer, and the Messenger of Allah ﷺ replied, "The night prayer is two units at a time. When one of you fears the approach of dawn, let him pray a single unit, which will make all that he has prayed odd-numbered."
أن رجلا سأل رسول الله ﷺ عن صلاة الليل فقال رسول الله ﷺ: صلاة الليل مثنى مثنى. فإذا خشي أحدكم الصبح فصلى ركعة واحدة توتر له ما قد صلى.
Malik narrated it in Prayer at Night (13), from Nafi and Abdullah bin Dinar, from Ibn Umar, and he mentioned it. Bukhari narrated it in Witr (990) and Muslim in Prayer of the Traveler (749), both of them through the route of Malik, with the same wording.