Muslims regard the Qur'an as the primary source of knowledge about the historical Muhammad. The Qur'an has a few allusions to Muhammad's life. The Qur'an responds "constantly and often candidly to Muhammad's changing historical circumstances and contains a wealth of hidden data."
Next in importance are the historical works by writers of the third and fourth century of the Muslim era. These include the traditional Muslim biographies of Muhammad and quotes attributed to him (the sira and hadith literature), which provide further information on Muhammad's life.
The earliest surviving written sira (biographies of Muhammad and quotes attributed to him) is Ibn Ishaq's Life of God's Messenger written ca. 767 (150 AH). The work is lost, but was used verbatim at great length by Ibn Hisham and Al-Tabari.
Another early source is the history of Muhammad's campaigns by al-Waqidi (death 207 of Muslim era), and the work of his secretary Ibn Sa'd al-Baghdadi (death 230 of Muslim era).
Many scholars accept the accuracy of the earliest biographies, though their accuracy is unascertainable. Recent studies have led scholars to distinguish between the traditions touching legal matters and the purely historical ones. In the former sphere, traditions could have been subject to invention while in the latter sphere, aside from exceptional cases, the material may have been only subject to "tendential shaping".
In addition, the hadith collections are accounts of the verbal and physical traditions of Muhammad that date from several generations after his death.Hadith compilations are records of the traditions or sayings of Muhammad. They might be defined as the biography of Muhammad perpetuated by the long memory of his community for their exemplification and obedience.
Western academics view the hadith collections with caution as accurate historical sources. Scholars such as Madelung do not reject the narrations which have been complied in later periods, but judge them in the context of history and on the basis of their compatibility with the events and figures.
Finally, there are oral traditions. Although usually discounted by historians, oral tradition plays a major role in the Islamic understanding of Muhammad.[
The earliest Greek source for Muhammed is the 9th century writer Theophanes. The earliest Syriac source is the 7th century John bar Penkaye.