Musnad Ahmad 5:661

Abdullah narrated to us, Uthman ibn Abi Shaybah narrated to me, Jarir narrated to us, from Mansur, from Abu Ishaq, from Asim ibn Damrah, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah is Witr (Odd/One) and He loves the Witr. So perform Witr, O people of the Quran."