Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon XXIII

Canon XXIII.  (Greek xxvi.)

That bishops shall not go across seas.

Item, That bishops shall not go beyond seas without consulting the bishop of the primatial see of his own province:  so that from him they may be able to receive a formed or commendatory letter.

Notes.

Ancient Epitome of Canon XXIII.

A bishop is not to cross the seas unless he has received from the Primate of his region a letter dimissory.

This is Canon xxvij. of Hippo, 393.

Johnson.

See note on Canons of the Apostles, 10 (13). [viz.:]

[The use of Letters Commendatory was very early in the Church; St. Paul mentions them 2 Cor. 3.1.  And it is not easy to be conceived how discipline can be restored but by the reviving of this practice.  It is surely irregular to admit all chance comers to the Communion, who, for aught we know, may stand excommunicated by their own bishop.  Of the difference between Commendatory and Pacific and Formal Letters, see Can. Chalc., 11; Apost., 25, 26; Ant., 6; Sardic., 13].