Jami` at-Tirmidhi 453
جامع الترمذي#453Sahihحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو بَكْرِ بْنُ عَيَّاشٍ، حَدَّثَنَا أَبُو إِسْحَاقَ، عَنْ عَاصِمِ بْنِ ضَمْرَةَ، عَنْ عَلِيٍّ، قَالَ الْوِتْرُ لَيْسَ بِحَتْمٍ كَصَلاَتِكُمُ الْمَكْتُوبَةِ وَلَكِنْ سَنَّ رَسُولُ اللَّهِ صلى الله عليه وسلم وَقَالَ " إِنَّ اللَّهَ وِتْرٌ يُحِبُّ الْوِتْرَ فَأَوْتِرُوا يَا أَهْلَ الْقُرْآنِ " . قَالَ وَفِي الْبَابِ عَنِ ابْنِ عُمَرَ وَابْنِ مَسْعُودٍ وَابْنِ عَبَّاسٍ . قَالَ أَبُو عِيسَى حَدِيثُ عَلِيٍّ حَدِيثٌ حَسَنٌ .
Ali said:"Al-Witr is not incumbent like your obligatory prayers, but it is a Sunnah of Allah's Messenger who said: "Indeed Allah is Witr (One), and He loves Al-Witr, so perform Al-Witr O people of the Qur'an
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 3:2