Proto-Slavic, sometimes referred to as Common Slavic or Late Proto-Slavic, is defined as the last stage of the language preceding the geographical split of the historical Slavic languages. That language was uniform, and on the basis of borrowings from foreign languages and Slavic borrowings into other languages, cannot be said to have any recognizable dialects, suggesting a comparatively compact homeland. Slavic linguistic unity was to some extent visible as late as Old Church Slavonic manuscripts which, though based on local Slavic speech of Thessaloniki, could still serve the purpose of the first common Slavic literary language.

Answer this question, if possible (if impossible, reply "unanswerable"): Old Church Slavonic manuscripts were based on the local Slavic speech of what?
Thessaloniki