Musnad Ahmad 27:768

Waki’ narrated to us, ‘Imran bin Hudair narrated to us, from ‘Abdullah bin Shaqiq al-‘Uqayli, from Ibn ‘Umar, who said: A man came to the Prophet, may Allah bless him and grant him peace, and asked him about the night prayer, and I was between the questioner and the Prophet, may Allah bless him and grant him peace. He said: "Two by two. If you fear the morning, then perform one rak'ah as witr." He said: Then he came at the turn of the year, and I was in that same position between him and the questioner, and he asked him. He said: "Two by two. If you fear the morning, then perform one rak'ah as witr."