Sunan an-Nasa'i, Hadith 1669 ⓘ
It was narrated that Ibn Umar said: "I heard the Messenger of Allah on the minbar, when he was asked about prayers at night, say: "Two by two, then if you fear that dawn will come, pray witr with one rak'ah.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ ابْنِ أَبِي لَبِيدٍ، عَنْ أَبِي سَلَمَةَ، عَنِ ابْنِ عُمَرَ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَلَى الْمِنْبَرِ يُسْأَلُ عَنْ صَلاَةِ اللَّيْلِ فَقَالَ " مَثْنَى مَثْنَى فَإِذَا خِفْتَ الصُّبْحَ فَأَوْتِرْ بِرَكْعَةٍ " .
also referenced under: Book 20, Hadith 72 (In-Book) & Volume 2, Book 20, Hadith 1670 (DarusSalam)