Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Fourth Ecumenical Council.  The Council of Chalcedon.: Canon XX

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.

Notes.

Ancient Epitome of Canon XX.

A clergyman of one city shall not be given a cure in another.  But if he has been driven from his native place and shall go into another he shall be without blame.  If any bishop receives clergymen from without his diocese he shall be excommunicated as well as the cleric he receives.

It is quite doubtful as to what “excommunication” means in this canon, probably not anathematism (so think the commentators) but separation from the communion of the other bishops, and suspension from the performance of clerical functions.

Bright.

This canon is the third of those which were originally proposed by Marcian in the end of the sixth session, as certain articles for which synodical sanction was desirable (see above Canons iij. and iv.).  It was after they had been delivered by the Emperor’s own hand to Anatolius of Constantinople that the Council broke out into plaudits, one of which is sufficiently startling, τῷ ἱερεῖ, τῷ βασιλεῖ (Mansi, vii., 177).  The imperial draft is in this case very slightly altered.  A reference is made to a previous determination (i.e., canon x.) against clerical pluralities, and it is ordered that “clerics registered as belonging to one church shall not be ranked as belonging to the church of another city, but must be content with the one in which they were originally admitted to minister, excepting those who, having lost their own country, have been compelled to migrate to another church,”—an exception intelligible enough at such a period.  Eleven years before, the Vandal Gaiseric had expelled the Catholic bishops p. 283 and priests of Western Africa from their churches:  Quodvultdeus, bishop of Carthage with many of his clergy, had been “placed on board some unseaworthy vessels,” and yet, “by the Divine mercy, had been carried safe to Naples” (Vict. Vitens., De Persec. Vandal., i., 5:  he mentions other bishops as driven into exile).  Somewhat later, the surge of the Hunnish invasion had frightened the bishop of Sirmium into sending his church vessels to Attila’s Gaulish secretary and had swept onward in 447 to within a short distance of the “New Rome” (Hodgkin, Italy and her Invaders, ii., 54–56).  And the very year of the Council was the most momentous in the whole history of the “Barbaric” movement.  The bishops who assembled in October at Chalcedon must have heard by that time of the massacre of the Metz clergy on Easter Eve, of a bishop of Rheims slain at his own altar, of the deliverance of Orleans at the prayer of St. Anianus, of “the supreme battle” in the plain of Chalons, which turned back Attila and rescued Christian Gaul (Hodgkin, ii., 129–152; Kitchin, Hist. France, i. 61).

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. lxxi, c. iv.