Sunan Ibn Majah, Volume 2, Book 5, Hadith 1319 (DarusSalam) ⓘ
It was narrated from Ibn ‘Umar that the Messenger of Allah said: “The night prayer is (to be offered) two by two.”
حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ، أَنْبَأَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى " .
also referenced under: Hadith 1319 (Reference) & Book 5, Hadith 1380 (In-Book)