History of Islam Part II

Muhammad and the Muslim era: from 622

The people of Yathrib, a prosperous oasis, welcome Muhammad and his followers. As a result, the move from Mecca in 622 comes to seem the beginning of Islam.

The Muslim era dates from the Hegira - Arabic for 'emigration', meaning Muhammad's departure from Mecca. In the Muslim calendar this event marks the beginning of year 1.

Yathrib is renamed Madinat al Nabi, the 'city of the prophet', and thus becomes known as Medina. Here Muhammad steadily acquires a stronger following. He is now essentially a religious, political and even military leader rather than a merchant (Khadija has died in 619).

He continues to preach and recite the words which God reveals to him. It is these passages, together with the earlier revelations at Mecca, which are written down in the Arabic script by his followers and are collected to become the Qur'an - a word (often transliterated as Quran) with its roots in the idea of 'recital', reflecting the oral origin of the text. The final and definitive text of the Qur'an is established under the third caliph, Othman, in about 650.

Source: http://www.historyworld.net/wrldhis/PlainTextHistories.asp?historyid=aa56#ixzz4G7c4Wd5f