Musnad Ahmad 457:9

Muhammad ibn Abdullah ibn al-Muthanna narrated to us, he said: Hisham ibn Abi Abdullah al-Dastawa'i narrated to us, he said: Hammad narrated to us, from Ibrahim, from Abu Abdullah al-Jadali, from Abu Mas'ud Uqbah ibn Amr al-Ansari, he said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform the Witr prayer at the beginning of the night, in the middle of it, and at the end of it."