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

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.

Notes.

Ancient Epitome of Canon XV.

Neither bishop, presbyter, nor deacon shall pass from city to city.  But they shall be sent back, should they attempt to do so, to the Churches in which they were ordained.

Hefele.

The translation of a bishop, priest, or deacon from one church to another, had already been forbidden in the primitive Church.  Nevertheless, several translations had taken place, and even at the Council of Nice several eminent men were present who had left their first bishoprics to take others:  thus Eusebius, Bishop of Nicomedia, had been before Bishop of Berytus; Eustathius, Bishop of Antioch, had been before Bishop of Berrhœa in Syria.  The Council of Nice thought it necessary to p. 33 forbid in future these translations, and to declare them invalid.  The chief reason of this prohibition was found in the irregularities and disputes occasioned by such change of sees; but even if such practical difficulties had not arisen, the whole doctrinal idea, so to speak, of the relationship between a cleric and the church to which he had been ordained, namely, the contracting of a mystical marriage between them, would be opposed to any translation or change.  In 341 the Synod of Antioch renewed, in its twenty-first canon, the prohibition passed by the Council of Nice; but the interest of the Church often rendered it necessary to make exceptions, as happened in the case of St. Chrysostom.  These exceptional cases increased almost immediately after the holding of the Council of Nice, so that in 382, St. Gregory of Nazianzum considered this law among those which had long been abrogated by custom.  It was more strictly observed in the Latin Church; and even Gregory’s contemporary, Pope Damasus, declared himself decidedly in favour of the rule of Nice.

This canon is found in the Corpus Juris Canonici. Decretum, Pars II. Causa VII, Q. 1, c. xix.