From Sahih Bukhari (Hadith 3489):
Narrated Ibn `Abbas:Regarding the Verse: 'And (We) made you into Shu'ub and Qabail-- (49.13) that Shu'ub means the big Qabail (i.e. nations) while the Qabail (i.e. tribes) means the branch tribes
Reference: Bukhari 3489