Rashi on Numbers 17:18/רש"י על במדבר י״ז:י״ח

1כי מטה אחד FOR ONE STAFF SHALL BE (FOR THE HEAD OF THE HOUSE OF THEIR FATHERS) — i.e. although I have divided them into two families, the family of the priests apart, and that of the Levites apart, yet it is only one tribe, [and they shall therefore have but one staff].