Musnad Ahmad 5:224

Hashim narrated to us, Abu Khaythama narrated to us, Abu Ishaq narrated to us, from Asim ibn Damra, from Ali, who said: "Indeed, the Witr is not obligatory, but it is a Sunnah from the Messenger of Allah, may Allah bless him and grant him peace. And indeed, Allah, the Almighty and Majestic, is Witr (One/Odd) and He loves the Witr."