Sunan Ibn Majah, Volume 2, Book 5, Hadith 1174 (DarusSalam) ⓘ
It was narrated that Ibn ‘Umar said: “The Messenger of Allah used to pray (voluntary prayers) at night two by two, and he would pray one Rak’ah of Witr.”
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدَةَ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ أَنَسِ بْنِ سِيرِينَ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ يُصَلِّي مِنَ اللَّيْلِ مَثْنَى مَثْنَى وَيُوتِرُ بِرَكْعَةٍ .
also referenced under: Hadith 1174 (Reference) & Book 5, Hadith 1230 (In-Book)