Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The First Ecumenical Council:  The First Council of Nice.: Canon XVI

Canon XVI.

Neither presbyters, nor deacons, nor any others enrolled among the clergy, who, not having the fear of God before their eyes, nor regarding the ecclesiastical Canon, shall recklessly remove from their own church, ought by any means to be received by another church; but every constraint should be applied to restore them to their own parishes; and, if they will not go, they must be excommunicated.  And if anyone shall dare surreptitiously to carry off and in his own Church ordain a man belonging to another, without the consent of his own proper bishop, from whom although he was enrolled in the clergy list he has seceded, let the ordination be void.

Notes.

Ancient Epitome of Canon XVI.

Such presbyters or deacons as desert their own Church are not to be admitted into another, but are to be sent back to their own diocese.  But if any bishop should ordain one who belongs to another Church without the consent of his own bishop, the ordination shall be cancelled.

“Parish” in this canon, as so often elsewhere, means “diocese.”

Balsamon.

It seemed right that the clergy should have no power to move from city to city and to change their canonical residence without letters dimissory from the bishop who ordained them.  But such clerics as are called by the bishops who ordained them and cannot be persuaded to return, are to be separated from communion, that is to say, not to be allowed to concelebrate (συνιερουργεῖν) with them, for this is the meaning of “excommunicated” in this place, and not that they should not enter the church nor receive the sacraments.  This decree agrees with canon xv. of the Apostolical canons, which provides that such shall not celebrate the liturgy.  Canon xvj. of the same Apostolical canons further provides that if a bishop receive a cleric coming to him from another diocese without his bishop’s letters dimissory, and shall ordain him, such a bishop shall be separated.  From all this it is evident that the Chartophylax of the Great Church for the time does rightly in refusing to allow priests ordained in other dioceses to offer the sacrifice unless they bring with them letters commendatory and dimissory from those who ordained them.

Zonaras had also in his Scholion given the same explanation of the canon.

This canon is found in the Corpus Juris Canonici, divided into two.  Decretum.  Pars II, Causa VII. Quæst. I. c. xxiij.; and Pars I. Dist. LXXI., c. iij.