Slavs first wrote with runes.

Archaeology magazine looks at a cow bone engraved with runic letters around 600 CE in the southern Czech Republic – a discovery that indicates that Slavic people first wrote using the letters of the Vikings:

The runes inscribed on the bone are in the Elder Futhark script, which was used by German speakers in central Europe from the second through the seventh centuries, according to Robert Nedoma of the University of Vienna. The last seven of the 24 signs in the Elder Futhark script were inscribed on the bone fragment. The international team of scientists who analyzed the bone concluded that it may have been used as a teaching tool.

Photo at the link.