Muhammad could not read or write so his followers recorded the suras after his death. Muhammad is described as unlettered in the Koran.
“Our signs believe; to those who shall follow the Apostle — the Unlettered Prophet — whom they shall find described to them in the Torah and the Gospel.” Sura 7:157
“Therefore have faith in God and His apostle, the Unlettered Prophet, who believes in God and His commandments.” Sura 7:158
By 652, 20 years after Muhammad’s death, the Koran was codified. Under Muhammad’s third successor, Uthman, all the variant readings of the Koran were burned. Because of this, even some within Islam claim the text was tampered with.
Much of the suras had been written on palm leaves, bones, leather, boards, tree bark, stones, etc. Some of these were broken or even eaten by animals. Some of the Koran was recorded only on the memories of Muhammad’s followers. Some of these men, who were the only ones that remembered certain suras, died in battle before these got written down. There was much confusion as the memories of different men disagreed as to what Muhammad had said.