Summarize this text:
Qur'an means recital and is the sacred book of Islam. Its literary quality is considered especially remarkable since Muhammad himself was probably illiterate. He recited the verses, and then they were memorized and written down by others. Usually God is speaking to humanity, though occasionally the messengers Gabriel and Muhammad speak. In the longest chapter "The Cow" believers are reminded to pray facing Mecca, give charity to the poor, and fast until sunset during the month of Ramadan. After two of the six Muslims, who had memorized the entire Qur'an, were killed, Muhammad's successor Abu Bakr (r. 632-634) ordered the written and oral revelations collected together. Qur'an reciters were sent out to teach Islam. The caliph 'Uthman (r. 644-656) had been a secretary of Muhammad, and during his reign the Qur'an was edited into its final form. The 114 chapters are ordered mostly by decreasing length. These selections are placed in their approximate chronological order.