Musnad Abu Ya'la al-Mawsili 3:325

Zuhayr narrated to us, Jarir narrated to us, from Mansur, from Abu Ishaq, from Asim ibn Damrah, from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah is Witr (Odd/One) and He loves the Witr, so perform Witr, O people of the Quran."