Musnad Ahmad 28:441

Muhammad ibn Sawa Abu al-Khattab al-Sadusi narrated to us, he said: I asked al-Muthanna ibn al-Sabah, regarding Amr ibn Shu’ayb, from his father, from his grandfather, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, Allah has added a prayer for you, so maintain it, and it is the Witr." And Amr ibn Shu’ayb was of the opinion that the Witr should be repeated, even if it were after a month.