Virtues of the Qur'an
Bukhari :: Book 6 :: Volume 61 :: Hadith 507
Narrated 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 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.
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