The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day
20/27. Chapter:
The command to pray witr
Sunan an-Nasai
أَخْبَرَنِي مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، عَنْ أَبِي نُعَيْمٍ، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَاصِمِ بْنِ ضَمْرَةَ، عَنْ عَلِيٍّ، رضى الله عنه قَالَ الْوِتْرُ لَيْسَ بِحَتْمٍ كَهَيْئَةِ الْمَكْتُوبَةِ وَلَكِنَّهُ سُنَّةٌ سَنَّهَا رَسُولُ اللَّهِ صلى الله عليه وسلم .
It was narrated that Ali, may Allah (SWT) be pleased with him, said: "Witr is not essential like the obligatory prayers, but it is the sunnah of the Messenger of Allah (ﷺ)."
Reference : Sunan an-Nasa'i 1676In-book reference : Book 20, Hadith 79English translation : Vol. 2, Book 20, Hadith 1677