سنن ابن ماجه

‘Ali bin Abu Talib said: “Witr is not definite (obligatory) nor is it like your prescribed prayers. But the Messenger of Allah (ﷺ) prayed Witr, then he said: ‘O people of the Qur’an! Perform Witr, for Allah is Witr* and He loves the odd (numbered).’”
* Meaning 'one' which is the first of the odd numbers; He is unique, and there is nothing like Him, similar or equal.