بُلُوغُ الْمَرَامِ
Bulugh al-Maram 375
The Book of Prayer
وَعَنْ عَبْدِ اَللَّهِ بْنِ بُرَيْدَةَ , عَنْ أَبِيهِ قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ اَلْوِتْرُ حَقٌّ, فَمَنْ لَمْ يُوتِرْ فَلَيْسَ مِنَّا } أَخْرَجَهُ أَبُو دَاوُدَ بِسَنَد لَيِّنٍ, وَصَحَّحَهُ اَلْحَاكِمُ . [1] .
Narrated 'Abdullah bin Buraidah (RA) from his father: Allah's Messenger (ﷺ) said: "The Witr is a duty, so he who does not offer it, is not one of us." [Reported by Abu Dawud with a Laiyin (weak) chain of narrators and Al-Hakim graded it Sahih (authentic)].
Sunnah.com reference: Book 2, Hadith 279