Imran bin Husain narrated that:
The Prophet was asked about Ash-Shafi, so he said: "It is As-Salat, some of it is Shaf (even) and some of it is Witr (odd)." (ID)
English reference (ID): Vol. 5, Book 44, Hadith 3342 (ID)
Arabic reference (ID): Book 47, Hadith 3665 (ID)