Musnad Ahmad 5:695

'Abdullah narrated to us, 'Abdullah ibn Sandal and Suwayd ibn Sa'id narrated to us, both of them—in the year two hundred and twenty-six—they said: Abu Bakr ibn 'Ayyash narrated to us, from Abu Ishaq, from 'Asim ibn Damrah al-Saluli, who said: 'Ali said: "Behold, the Witr is not an obligation like your prescribed prayers, but the Messenger of Allah (peace and blessings of Allah be upon him) performed Witr, then said: 'Perform Witr, O people of the Qur'an, perform Witr, for Allah is Witr (One/Odd) and He loves the Witr.'" This is the wording of the hadith of 'Abdullah ibn Sandal, and their meaning is one.