Rashi on Deuteronomy 10:8/רש"י על דברים י׳:ח׳

1בעת ההוא הבדיל ה׳ וגו׳ AT THAT TIME THE LORD SEPARATED [THE TRIBE OF LEVI] — This is to be connected with the former narrative (that which speaks of the tablets of stone, ending at v. 5; vv. 6—8 which contain reference to Aaron’s death being interpolated for the reason given by Rashi);

2בעת ההוא AT THAT TIME accordingly means: In the first year of the Exodus from Egypt, when ye sinned by worshipping the golden calf, but the sons of Levi did not thus sin. — at that time God separated them from you. It places this verse in juxtaposition with the retreat to Bene Jaakon to tell you that in this matter also, the sons of Levi did not sin, but stood steadfast in their faith.

3לשאת את ארון TO BEAR THE ARK — [This was the function of] the Levites (such as were not כהנים).

4‎ לעמד לפני ה׳ לשרתו ולברך בשמו TO STAND BEFORE THE LORD, TO MINISTER UNTO HIM AND TO BLESS IN HIS NAME — This (to bless in His Name) was the function of the priests and refers to the “raising of the hands” (a technical term for reciting the priestly benediction which is done with uplifted hands) (Arakhin 11a).