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.
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. 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). 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.
Source: Oral tradition – Wikipedia