Getica (The Origin and Deeds of the Goths) by Jordanes 551 CE

"De origine actibusque Getarum (The Origin and Deeds of the Getae [Goths], commonly abbreviated Getica, written in Late Latin by Jordanes in or shortly after 551 AD, claims to be a summary of a voluminous account by Cassiodorus of the origin and history of the Gothic people, which is now lost. However, the extent to which Jordanes actually used the work of Cassiodorus is unknown. It is significant as the only remaining contemporaneous resource that gives the full story of the origin and history of the Goths. Another aspect of this work is its information about the early history and the customs of the Slavs."

-summary taken from wikipedia

Source:

https://people.ucalgary.ca/~vandersp/Courses/texts/jordgeti.html