It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) was asked about the even and the odd (al-shaf' wa al-watr), and he (peace and blessings be upon him) said: "It is the prayer: some of it is even and some of it is odd."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15669