Musnad Ahmad 5:665

'Abdullah narrated to us, 'Amr ibn Muhammad ibn Bukayr al-Naqid narrated to me, 'Abdullah ibn Dawud al-Khuraybi narrated to us, from 'Ali ibn Salih, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: "Indeed, the Witr is not an obligation, but it is a Sunnah which the Messenger of Allah, peace and blessings of Allah be upon him, established. So perform the Witr, O people of the Qur'an."