Belarusians and Russians are one people living in different states. And one sees the proof for that practically every day.
A recurring pro-Kremlin narrative denying the Belarusian statehood and nation. See more cases here.
This is an Imperial Russian and Russian irredentist ideology which is aimed at weakening the national identity of Belarus. Although Belarusian and Russian people share parts of their history, Belarus is an independent nation-state with a long history. Belarus preserved a different language, literature and identity, despite foreign rule for a long period on its territory.