To whom did Hashem say "Hibadlu"?
Ramban #1 (citing R. Chananel): He said it to the whole of Yisrael. 1
To whom does "ha'Eidah ha'Zos refer?
Ramban #1 (citing R. Chananel): It refers to Korach and his congregation.
Ramban #2: It refers to all of Yisrael, who were dithering between Korach and Moshe, and were therefore Chayav Misah bi'Yedei Shamayim. 1
Ramban: For doubting their Rebbe's veracity (See Sanhedrin, 110a) and for negating the prophecy of a Navi.
Why did Hashem instruct Moshe and Aharon to leave the vicinity of the congregation of Korach?