Sunan Ibn Majah, Hadith 1318 ⓘ
It was narrated that Ibn ‘Umar said: “The Messenger of Allah used to offer the night prayers two by two.”
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدَةَ، أَنْبَأَنَا 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 .
also referenced under: Book 5, Hadith 1379 (In-Book) & Volume 2, Book 5, Hadith 1318 (DarusSalam)