Rashi on Numbers 1:49/רש"י על במדבר א׳:מ״ט

1אך את מטה לוי לא תפקד ONLY THOU SHALT NOT NUMBER THE TRIBE OF LEVI — The legion of the King is worthy to be numbered by itself (cf. Midrash Tanchuma, Bamidbar 15). — Another explanation is: The Holy One, blessed be He, foresaw that a decree would once be made against all those that had been numbered from twenty years and upwards, viz., that they should die in the wilderness (Numbers 14:29). He therefore said: Let these (the Levites) not be included amongst those now counted, in which case they must die, because they are Mine, since they did not sin by worshipping the golden calf (cf. Numbers Rabbah 3:7; Bava Batra 121b).