Musnad Ahmad 27:677

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj said: Shu'bah narrated to me, from Qatadah, and I heard Abu Mijlaz, I heard Ibn 'Umar narrating from the Prophet, may Allah bless him and grant him peace, that he said: "The Witr is one rak'ah from the end of the night."