Musannaf Ibn Abi Shayba 3:4474

Abu Khalid al-Ahmar narrated to us, from Hajjaj, from Abu Ishaq, from Asim, from Ali, who said: It was said to him, "Is the Witr a mandatory duty?" He said: "The Prophet, peace and blessings of Allah be upon him, performed the Witr, and the Muslims have remained steadfast upon it."