hi! there's room for more input on this, but fyi, there have been a few discussions on this question before; check them out
There different kinds of evidence and that fact was disputed by Russian slavophiliacs starting with the 19th century. They wanted the Varangians to be another Slavic nation rather than vikings.
What evidence do we have for Nordicist theory?
Askold, Dir, Rurik, Olga, Oleg (mentioned as "HLGU" by the Khazars) are Nordic names. After a couple of Generations the Russian dukes would settle for Slavic ones, like "Yaroslav".
Contemporary foreigners identified them as Northmen.
Ibn Rustah describes the Rus as raiders living from the produce of their Saqaliba slaves. Saqaliba is a term he also used for the Slavs in Croatia (10th century).
Ibn Fadlan described pretty much your stereotypical vikings. (also 10th century)
In the primary chronicle by Nestor (writing in Kiev in 1113) Rurik is identified as a Varangians who have arrived from Sweden.
the law of Yaroslav (1016) is a list of Weregild ("Vira") sums to be paid dependant on the status of the victim.
The Finnish word for Swedes today is "Ruotsi". The Russians are called "Venäläiset".
The Slavophiles offered alternative interpretations for the names and ethnonyms used in ancient chronicles because we don't have any hard proof they were Scandis. They didn't leave any runestones with scandi language inscriptions; There is just this monk Nestor explicitly stating that for a fact as if he knows better than anyone else. They said that all the evidence we have is circumstantial.