Ali bin Abu Talib reported,
The witr prayer is not obligatory, nor is it like your prescribed prayers. However, the Messenger of Allah ﷺ performed the witr and then said, "People of the Quran, perform the witr, for Allah is One and loves what is odd in number."
Ibn Majah 1169
Daifحدثنا علي بن محمد ومحمد بن الصباح قالا حدثنا أبو بكر بن عياش عن أبي إسحق عن عاصم بن ضمرة السلولي قال
قال علي بن أبي طالب
إن الوتر ليس بحتم ولا كصلاتكم المكتوبة ولكن رسول الله ﷺ أوتر ثم قال يا أهل القرآن أوتروا فإن الله وتر يحب الوتر
[Its chain of transmission is daif] Abu Dawud narrated it in the chapters on Witr, chapter on the recommendation of Witr, no. 1416, from the hadith of Abu Ishaq with this wording, and Tirmidhi graded it hasan. See volume 4:6. I did not find an explicit statement that Abu Ishaq heard it directly, and it has corroborating narrations, all of which are daif. Ahmad narrated it in 1/107 with a sahih chain, from Abu Ishaq, who said, "I heard Asim bin Damra narrating from Ali 
," who said, "Witr is not obligatory like the prescribed prayer, but it is a tradition, so do not abandon it." Shubah said, "I found it written down in my possession, that the Messenger of Allah ﷺ used to perform Witr." Its chain of transmission is hasan.