Council of Chalcedon, Canon XX

It shall not be lawful, as we have already decreed, for clergymen officiating in one church to be appointed to the church of another city, but they shall cleave to that in which they were first thought worthy to minister; those, however, being excepted, who have been driven by necessity from their own country, and have therefore removed to another church.  And if, after this decree, any bishop shall receive a clergyman belonging to another bishop, it is decreed that both the received and the receiver shall be excommunicated until such time as the clergyman who has removed shall have returned to his own church.

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