There is no short prooftext of that, however, the logic of the letter is that the Church Catholic is likened to the priestly system established by Moses, and in the priestly system of Moses there was a chief priest Aaron. The call is then made to submit to that system and avoid the sedition.
“The call is then made to submit to that system and avoid the sedition.”
A “system” yes; the Fathers are surprisingly uniform about that; but the Pope? No mention anywhere, my brother.