Analysis of information sources in references of the Wikipedia article "Demographic history of Macedonia" in English language version.
Though some Slavic incursions had occurred earlier, the number of raids increased in the second half of Justinian's reign. [...] Procopius also speaks of Slavs living in the vicinity of certain Thracian cities, suggesting that by the middle of the sixth century a certain amount of Slavic settlement (as opposed to raiders coming, raiding, and then returning beyond the Danube) had begun in the Balkans.