بُلُوغُ الْمَرَامِ
Bulugh al-Maram 371
The Book of Prayer
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]
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)].
Sunnah.com reference: Book 2, Hadith 275