Musnad Ahmad 980:3

Isma'il ibn Ibrahim narrated to us, he said: Al-Dastawa'i informed us, and Yazid informed us, Al-Dastawa'i informed us, Hammad narrated to us, from Ibrahim, from Abu 'Abd Allah al-Jadali, from 'Uqbah ibn 'Amr Abu Mas'ud, from the Prophet, peace and blessings of Allah be upon him: "That he used to perform the Witr prayer at the beginning of the night, its middle, and its end."