Musnad Ahmad 969
It was narrated that ‘Ali (رضي الله عنه) was asked about Witr and whether it was obligatory. He said:
It is not like the obligatory prayer, but it is a Sunnah that was done by the Messenger of Allah (ﷺ) and his Companions, and they continued doing it until the end of their lives.
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.‏