Numbers 17:3

And thou shalt write Aaron’s name upon the rod of Levi. — Aaron was descended from the second son of Levi. He was not, therefore, the natural, but the divinely-appointed head of his father’s house, and hence it would not have sufficed for the purpose contemplated to have inscribed the name of Levi upon the rod. Aaron was constituted the head alike of the priests and of the Levites, into which two classes the tribe of Levi was divided.