Virtues and Merits of the Prophet (pbuh) and his Companions
حَدَّثَنَا خَالِدُ بْنُ يَزِيدَ الْكَاهِلِيُّ، حَدَّثَنَا أَبُو بَكْرٍ، عَنْ أَبِي حَصِينٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسِ، رضى الله عنهما. {وَجَعَلْنَاكُمْ شُعُوبًا وَقَبَائِلَ} قَالَ الشُّعُوبُ الْقَبَائِلُ الْعِظَامُ، وَالْقَبَائِلُ الْبُطُونُ.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
From Sahih al-Bukhari, whose reports the scholars accept as authentic.