The Messenger of Allah ﷺ said, "The night prayer is to be performed two units at a time."
حدثنا محمد بن رمح أنبأنا الليث بن سعد عن نافع
أن رسول الله ﷺ قال صلاة الليل مثنى مثنى
No references are available.
Bukhari narrated it in the Book of Witr, Chapter on What Has Been Said Regarding Witr, no. 990, and Muslim narrated it in the Book of the Prayer of Travelers, Chapter on the Night Prayer Being Performed Two by Two and Witr Being One Rakat at the End of the Night, no. 749, from the hadith of Malik, from Nafi and others, in longer form. It has multiple routes of transmission in both of their collections.