Musnad Ahmad 790:105

Abu Dawud narrated to us, Hammam narrated to us, from Qatadah, from Imran ibn Isam, that an old man from the people of Basra narrated to him, from Imran ibn Husayn, that the Messenger of Allah (peace and blessings of Allah be upon him) was asked about the *Shaf'* (even) and the *Witr* (odd). He said: "It is the prayer; some of it is even, and some of it is odd."