Mishkat al-Masabih
Buraida said he heard God’s Messenger say, “The witr is a duty, so he who does not observe it does not belong to us; the witr is a duty, so he who does not observe it does not belong to us; the witr is a duty, so he who does not observe it does not belong to us.” Abu Dawud transmitted it.
وَعَن بُرَيْدَة قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «الْوَتْرُ حَقٌّ فَمَنْ لَمْ يُوتِرْ فَلَيْسَ مِنَّا الْوَتْرُ حَقٌّ فَمَنْ لَمْ يُوتِرْ فَلَيْسَ مِنَّا الْوَتْرُ حَقٌّ فَمَنْ لَمْ يُوتِرْ فَلَيْسَ مِنَّا» . رَوَاهُ أَبُو دَاوُدَ
Reference