In the introduction to his Sunan imam Abu Dawud (817-888) wrote a short epistle (risalah) to the people of Makkah on the history and methodology of the science of hadith. The Arabic version of the epistle can be downloaded here: http://read.kitabklasik.co.cc/2010/02/risalah-abi-dawud-ila-ahli-makkah-fi.html

Here the link to the English translation: http://ubd.academia.edu/mn/Papers/108119/Translation_of_the_Introduction_to_Sunan_Abu_Dawud