Mishkat al-Masabih 4 Prayer كتاب الصلاة 670

وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «الْوَتْرُ رَكْعَةٌ مِنْ آخر اللَّيْل» . رَوَاهُ مُسلم

He reported God’s Messenger as saying, “The witr * is a rak’a at the end of the night.” * Literally ‘single’, or ‘odd’, used of an odd number of rak’as prayed at some time during the night. Muslim transmitted it.