Sunan an-Nasa'i, Hadith 1667 ⓘ
It was narrated that Tawus said: "Ibn Umar said: "A man asked the Messenger of Allah about prayer at night. He said: "Two by two, and if you fear that dawn will come, then one.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ قُدَامَةَ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ حَبِيبٍ، عَنْ طَاوُسٍ، قَالَ قَالَ ابْنُ عُمَرَ سَأَلَ رَجُلٌ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ صَلاَةِ اللَّيْلِ فَقَالَ " مَثْنَى مَثْنَى فَإِذَا خَشِيتَ الصُّبْحَ فَوَاحِدَةٌ " .
also referenced under: Book 20, Hadith 70 (In-Book) & Volume 2, Book 20, Hadith 1668 (DarusSalam)