It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said about the even and the odd (al-shaf' wa al-watr): "It is the prayer: some of it is even and some of it is odd."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15670