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)].

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]‏ .‏