The revelations that comprise the Koran were revealed over a period of more than two decades in two places. The first revelations from the period of Muhammad's residence in Mecca are short and incantatory verses of extraordinary poetic beauty. The later revelations from the period after Muhammad immigrated to Medina are longer, legalistic texts appropriate to a developing community of believers in need of rules and regulations.

Muhammad and his followers initially committed the revelations to memory, but as these revelations grew in number and complexity, some were probably written down on whatever materials were at hand. After the Prophet died, his followers were pressed to preserve the purity of the revelations and began to write down as many of them as possible. According to the traditional view, a uniform written text of the revelations to Muhammad was collected and collated some twenty years after his death.

The Koran as a book is comparable in length to the Gospels. It contains 114 chapters (each called in Arabic a sura) of varying length. It opens with the Fatiha, a beautiful short prayer that serves as an invocation in many situations;

In the Name of God, the Merciful, the Compassionate
Praise belongs to God, Lord of all Being
the All-merciful, the All-compassionate
the Master of the Day of Doom
Thee only we serve; to Thee alone we pray for succor
Guide us in the straight path
the path of those whom Thou hast blessed,
not of those against whom Thou art wrathful
nor of those who are astray.

The other chapters of the Koran follow in descending order of length, from the 286 verses of the second chapter, known as "The Cow," to the final two chapters, which are short prayers of a few lines. The chapters are thus arranged neither in the order in which the verses were revealed nor in a narrative sequence.

The Koran, as God's literal word, can only be comprehended in the majestic and glorious Arabic language in which it was revealed. The necessity of reading the Koran in Arabic has meant that all believers should learn the language in order to understand the scriptures. This requirement has created a linguistic bond among believers, particularly as Islam spread beyond the boundaries of Arabia to regions inhabited by speakers of other languages. Having learned to use Arabic as the language of religion, the new converts also used it as a language of literature, science, commerce and social intercourse.

The primacy of Arabic as the language of God's revelation has also helped to preserve the purity of the Arabic language, for Muslims constantly call to mind the noble and magnificent words and phrases of the Koran. Although the Arabic language has evolved over the fourteen centuries since the Koran was revealed, it has not changed as much as English has changed in the six centuries since the time of Chaucer. Finally, the primacy of the Arabic language has encouraged the spread and use of the Arabic script, which is known and used from the shores of the Atlantic to the Pacific to render a variety of languages, including Arabic, Persian, Kurdish, Pashto, Kashmiri, Urdu, Sindhi, Ottoman Turkish, Chaghatay, and Malay.

The second basis of Muslim faith is the example of the Prophet. As the perfect Muslim, Muhammad served and still serves as the model for all believers. His sayings and deeds were remembered by his associates and preserved in the Traditions, known in Arabic as hadith. These Traditions normally take the form of a chain ("So-and-so heard from so-and-so, who heard from so-and-so, that the Prophet said [or did]"), followed by a report of what the Prophet said or did.

The Traditions came to be considered second in authority to the Koran and also help explain and elaborate the circumstances under which obscure passages in the Koran were revealed. The Traditions were transmitted orally for several generations before being written down, beginning in the eighth century. By the ninth century the jurist al-Shafii (d. 820) came to consider the sunna, or custom of the Prophet, the second most important root of Islamic jurisprudence after the Koran. Together the Koran and the Traditions, along with consensus and analogy, make up the sharia, the rules and regulations that govern the day-to-day lives of Muslims.