The Fourth Ecumenical Council.  The Council of Chalcedon 25Canon X.

It shall not be lawful for a clergyman to be at the same time enrolled in the churches of two cities, that is, in the church in which he was at first ordained, and in another to which, because it is greater, he has removed from lust of empty honour. And those who do so shall be returned to their own church in which they were originally ordained, and there only shall they minister. But if any one has heretofore been removed from one church to another, he shall not intermeddle with the affairs of his former church, nor with the martyries, almshouses, and hostels belonging to it. And if, after the decree of this great and ecumenical Synod, any shall dare to do any of these things now forbidden, the synod decrees that he shall be degraded from his rank.