Terracotta tile depicting battle between Bulgars and Slavs 7th C. CE

"...on the other hand, the Bulgars have lost their original Turkish language; they now speak the language of the Slav peoples, to the south of the lower Danube, on whom the Bulgars imposed their rule in and after the year 680".

-Arnold Toynbee, Constantine Porphyrogenitus and his World, page 428

Terracotta tile from the 7th century CE found in Vinica, North Macedonia depicts a battle scene between the Bulgars and Slavs with the Latin inscription BOLGAR and SCLAVIGI.

Source:

https://commons.wikimedia.org/wiki/File:Bolgari_sclavi_teracota_Vinitza_FYROM.jpg

https://commons.wikimedia.org/wiki/File:BOLGAR_icon_Vinitza.jpg

Quote:

Arnold Toynbee, Constantine Porphyrogenitus and his World, page 428