كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 371
Narrated 'Ali bin Abu Talib (RA):
The Witr prayer is not obligatory as the prescribed prayer is [i.e. the five daily prayers], but it is a Sunnah which was established by the practice of Allah's Messenger (ﷺ). [at-Tirmidhi and an-Nasa'i reported it, who graded it Hasan (good). al-Hakim graded it Sahih (authentic)].
وَعَنْ عَلِيِّ بْنِ أَبِي طَالِبٍ ‏- رضى الله عنه ‏- قَالَ : { لَيْسَ اَلْوِتْرُ بِحَتْمٍ كَهَيْئَةِ اَلْمَكْتُوبَةِ , وَلَكِنْ سُنَّةٌ سَنَّهَا رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-} رَوَاهُ النَّسَائِيُّ وَاَلتِّرْمِذِيُّ وَحَسَّنَهُ وَالْحَاكِمُ وَصَحَّحَهُ .‏ 1
Reference : Bulugh al-Maram 371
In-book reference : Book 2, Hadith 371
English translation : Book 2, Hadith 366
Hadith Text
Reference