حَدَّثَنَا أَبُو مُعَاوِيَةَ، حَدَّثَنَا الْحَجَّاجُ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَاصِمِ بْنِ ضَمْرَةَ، عَنْ عَلِيٍّ
رَضِيَ اللَّهُ عَنْهُ قَالَ سُئِلَ عَنْ الْوَتْرِ، أَوَاجِبٌ هُوَ قَالَ: أَمَّا كَالْفَرِيضَةِ فَلَا وَلَكِنَّهَا سُنَّةٌ صَنَعَهَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَأَصْحَابُهُ حَتَّى مَضَوْا عَلَى ذَلِكَ
It is not like the obligatory prayer, but it is a Sunnah that was done by the Messenger of Allah (sallallahu alayhi wa sallam) and his Companions, and they continued doing it until the end of their lives.
Musnad Ahmad 921, Book 5, Musnad 'Ali Ibn Abi Talib, Hadith 393. 1 min read