Primary Chronicles:
"The work’s name originates from the opening sentence of the text, which reads: “These are the narratives of bygone years regarding the origin of the land of Rus’ (Old East Slavic: Рѹсь), the first princes of Kyiv, and from what source the land of Rus’ had its beginning.”[5]: 51 The work has long been considered to be a fundamental source in the interpretation of the history of the East Slavs."
https://en.wikipedia.org/wiki/Primary_Chronicle