The Canons of the Council in Trullo 20
20 Canon XX
It shall not be lawful for a bishop to teach publicly in any city which does not belong to him. If any shall have been observed doing this, let him cease from his episcopate, but let him discharge the office of a presbyter.
Notes.
Ancient Epitome of Canon XX.
The bishop of one city shall not teach publicly in another. If he shall be shown to have dose so he shall be deprived of the episcopate and shall perform the functions of a presbyter.
The meaning of this canon is most obscure. Balsamon and Zonaras think that the Bishop is not to be deposed from his Episcopate, but only shorn of his right of executing the Episcopal functions, so that he will virtually be reduced to a presbyter. Aristenus, on the other hand, considers the deposition to be real and that this canon creates an exception to Canon XXIX. of Chalcedon.