Musnad Ahmad 27:1730

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Nafi' narrated to me, from Ibn 'Umar, that when he was asked about the Witr, he said: "As for me, if I were to perform Witr before I sleep, and then I wanted to pray at night, I would make my past Witr even with one rak'ah, then I would pray in pairs. Then, when I finished my prayer, I would perform Witr with one rak'ah." "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) commanded that the last of the night prayer be the Witr."