(2) باب نَزَلَ الْقُرْآنُ بِلِسَانِ قُرَيْشٍ وَالْعَرَبِ
Narrated Anas bin Malik:
(The Caliph `Uthman ordered Zaid bin Thabit, Sa`id bin Al-As, `Abdullah bin Az-Zubair and `Abdur- Rahman bin Al-Harith bin Hisham to write the Qur'an in the form of a book (Mushafs) and said to them. "In case you disagree with Zaid bin Thabit (Al-Ansari) regarding any dialectic Arabic utterance of the Qur'an, then write it in the dialect of Quraish, for the Qur'an was revealed in this dialect." So they did it.
Reference: Sahih al-Bukhari 4984
In-book reference: Book 66, Hadith 6
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 6, Book 61, Hadith 507