Musnad Ahmad 5:647

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