Musnad Ahmad 790:121

Bahz narrated to us, Hammam narrated to us, he said: Qatadah was asked about the *Shaf'* (even) and the *Witr* (odd). He said: Imran bin 'Isam al-Dab'i narrated to us, from a sheikh from the people of Basra, from Imran bin Husayn, that the Prophet (peace and blessings of Allah be upon him) said: "It is the prayer: some of it is *Shaf'*, and some of it is *Witr*."