I did find that the Catholic Encyclopedia entry on Mohammed says:
The sources of Mohammed's biography are numerous, but on the whole untrustworthy, being crowded with fictitious details, legends, and stories. None of his biographies were compiled during his lifetime, and the earliest was written a century and a half after his death. The Koran is perhaps the only reliable source for the leading events in his career. His earliest and chief biographers are Ibn Ishaq (A.H. 151=A.D. 768), Wakidi (207=822), Ibn Hisham (213=828), Ibn Sa'd (230=845), Tirmidhi (279=892), Tabari (310-929), the "Lives of the Companions of Mohammed", the numerous Koranic commentators [especially Tabari, quoted above, Zamakhshari 538=1144), and Baidawi (691=1292)], the "Musnad", or collection of traditions of Ahmad ibn Hanbal (241=855), the collections of Bokhari (256=870), the "Isabah", or "Dictionary of Persons who knew Mohammed", by Ibn Hajar, etc. All these collections and biographies are based on the so-called Hadiths, or "traditions", the historical value of which is more than doubtful.
These traditions, in fact, represent a gradual, and more or less artificial, legendary development, rather than supplementary historical information....
If the accepted story of Mohammed's life was simply developed over time as a legend in service of furthering Islam, it shines a different light on events such as Mohammed's massacre of the Jews in Yathrib (before it was renamed Medina, the City of the Prophet), or his marriage to the six-year old girl Aisha, consummated when she was nine. If his life is a created legend, did the early Muslims think that these stories would illustrate the holiness of Mohammed's character?