Musnad Ahmad 27:1005

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Yahya, from Abu Salamah and Nafi’, the freed slave of Ibn Umar, that Ibn Umar informed him that the Messenger of Allah (ﷺ) said: "The night prayer is two units by two units. So, if you fear the morning, then perform one unit as Witr."