"Prehistoric Slavic is the group of Slavic dialects and/or languages in use prior to the advent of written Slavic languages. These dialects were not unified in appearance , and , by most accounts , at least five centuries separate the first differentiation of an ur - Slavic language into dialects from the first written Slavic texts. Following Slavistic convention, forms reconstructed for prehistoric Slavic are given in the Latin alphabet and marked with an asterisk. The earliest phase of this period - when all Slavs spoke one mutually intelligible language or group of dialects - can be called Proto-Slavic or Common Slavic"
https://www.google.co.uk/books/edition/Context_and_the_Lexic...

"It asserts that the ancient Macedonians were a pure Aryan people, whose language was the "ur-Slavic" tongue from which other Slavic languages later developed, having been taken by the inferior people used as farmhands by the original noble Macedonians."
https://www.rferl.org/a/1342558.html

"The alphabet spread from early medieval Bulgaria to other Slavic civilizations. With substantial justification, Bulgarians consider their native tongue the ur-Slavic language that influenced all the other Slavic languages"
https://www.encyclopedia.com/social-sciences/encyclopedias-a...

"Similar examples can be offered from languages all over the world. We do not, for instance, refer to Russian as “Modern Slavic” because Polish, Czech, Ukrainian, Serbian, etc., descend from the same ur-Slavic tongue. "
https://mosaicmagazine.com/observation/israel-zionism/2020/0...