The Prophet himself, sent hundreds of letters. Many of them were vey lengthy, containing the formulae of prayers and worship. According to the Qur'an the Prophet's conduct and deeds should be followed by the community. The Qur'an itself demands a record of credit transactions. Therefore it looks as if there were no general instructions not to record the ahadith, though it might have been explained by some of the scholars in this way.
On the other hand there is clear evidence to show that the Prophet approved of recording ahadith. Furterh we find that queite a number of companions recorded ahadith and among them were those people who are responsible for transmitting the hadith which forbade its recording. Bearing all this in mind one arrives at the conclusion that the Prophet's disapproval of writing ahadith most probably meant the writing of the Qur'an and non-Qur'anic material on the same sheet because that might have led to misunderstanding. There is another theory that people were forbidden to write down ahadith in early days because all attention should be paid to the Qur'an and its preservation, and that later on, when there was no danger of neglecting the Qur'an, the previous order was abrogated and permission was given to write ahadith.
From Chapter II - Recording of Ahadith
M. Mustafa Azmi