Sahih Bukhari | Volume 6 | Hadith Number 507
Narrated by Anas bin Malik
(The Caliph 'Uthman ordered Zaid bin Thabit, Said 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.