First Council of Nicaea, Canon XV

On account of the great disturbance and discords that occur, it is decreed that the custom prevailing in certain places contrary to the Canon, must wholly be done away; so that neither bishop, presbyter, nor deacon shall pass from city to city.  And if any one, after this decree of the holy and great Synod, shall attempt any such thing, or continue in any such course, his proceedings shall be utterly void, and he shall be restored to the Church for which he was ordained bishop or presbyter.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.