(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 Quran 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 Quran, then write it in the dialect of Quraish, for the Quran was revealed in this dialect." So they did it.
Sahih al-Bukhari Book 66, Hadith 6