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.
حَدَّثَنَا أَبُو الْيَمَانِ، حَدَّثَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، وَأَخْبَرَنِي أَنَسُ بْنُ مَالِكٍ، قَالَ فَأَمَرَ عُثْمَانُ زَيْدَ بْنَ ثَابِتٍ وَسَعِيدَ بْنَ الْعَاصِ وَعَبْدَ اللَّهِ بْنَ الزُّبَيْرِ وَعَبْدَ الرَّحْمَنِ بْنَ الْحَارِثِ بْنِ هِشَامٍ أَنْ يَنْسَخُوهَا، فِي الْمَصَاحِفِ وَقَالَ لَهُمْ إِذَا اخْتَلَفْتُمْ أَنْتُمْ وَزَيْدُ بْنُ ثَابِتٍ فِي عَرَبِيَّةٍ مِنْ عَرَبِيَّةِ الْقُرْآنِ فَاكْتُبُوهَا بِلِسَانِ قُرَيْشٍ، فَإِنَّ الْقُرْآنَ أُنْزِلَ بِلِسَانِهِمْ فَفَعَلُوا.
Bukhari
Virtues of the Qur'an • Hadith 6