Musnad Ahmad 27:636

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, who said: A man called out to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, how do you command us to pray at night?" He said: "One of you should pray two by two. Then, if he fears the morning, he should pray one [rak'ah], and it will make witr for him what he has already prayed."