(20) The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
كتاب قيام الليل وتطوع النهار
(26) Chapter: How to pray at night
(26) باب كَيْفَ صَلاَةُ اللَّيْلِ
Sunan an-Nasa'i 1672 
It was narrated that Ibn Umar said:
"A man from among the Muslims asked the Messenger of Allah (ﷺ): 'How are prayers at night to be done?' He said: 'prayers at night are (offered) two by two, then if you fear that dawn will come, pray witr with one.'"
أَخْبَرَنَا أَحْمَدُ بْنُ مُحَمَّدِ بْنِ الْمُغِيرَةِ، قَالَ حَدَّثَنَا عُثْمَانُ، عَنْ شُعَيْبٍ، عَنِ الزُّهْرِيِّ، عَنْ سَالِمٍ، عَنِ ابْنِ عُمَرَ، قَالَ سَأَلَ رَجُلٌ مِنَ الْمُسْلِمِينَ رَسُولَ اللَّهِ صلى الله عليه وسلم كَيْفَ صَلاَةُ اللَّيْلِ فَقَالَ ‏ "‏ صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا خِفْتَ الصُّبْحَ فَأَوْتِرْ بِوَاحِدَةٍ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 1672
In-book reference : Book 20, Hadith 75
English translation : Vol. 2, Book 20, Hadith 1673