The written Quran is said to have been created in part from what had been memorized by Muhammad’s companions, and the decision to create a standard written work is said to have come after the death in battle (Yamama) of a large number of Muslims who had memorized the work.[58]

For centuries copies of the Qurans/mushaf were written by hand, not printed, and their scarcity and expense made reciting the Quran from memory, not reading, the predominant mode of teaching it to others.[60] To this day the Quran is memorized by millions and its recitation can be heard throughout the Muslim world from recordings and mosque loudspeakers (during Ramadan).[60][61] Muslims state that some who teach memorization/recitation of the Quran constitute the end of an “un-broken chain” whose original teacher was Muhammad himself.[60]

Source: Oral tradition – Wikipedia