Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon XVIII

Canon XVIII.

Those clerics who in consequence of a barbaric incursion or on account of any other circumstance have gone abroad, we order to return again to their churches after the cause has passed away, or when the incursion of the barbarians is at an end.  Nor are they to leave them for long without cause.  If anyone shall not have returned according to the direction of this present canon—let him be cut off until he shall return to his own church.  And the same shall be the punishment of the bishop who received him.

Notes.

Ancient Epitome of Canon XVIII.

Whoever has emigrated on account of an invasion of the barbarians, shall return to the Church to whose clergy he belongs as soon as the incursion ceases.  But if he shall not do so, he shall be cut off together with him to whom he has gone.

Balsamon.

The Fathers are worthy of great praise.  For having regard to the honour of the ecclesiastical order and of each bishop, they have decreed that clergymen, who from just and valid causes have gone forth without letters dimissory from those who ordained them, should return to their own clergy soon as the cause which drove them forth ceases; and that they should not be enrolled on the clergy list of any other church.  But whosoever cannot be persuaded to return is to be cut off, as well as the bishop who detains him.  But someone will say, If a bishop who does such a thing is cut off by his Metropolitan; and likewise if a Metropolitan spurns this canon he is punished by the Patriarch.  But if an autocephalous archbishop or a Patriarch other than the Patriarch of Constantinople (for he has a faculty for doing so) should be convicted of a breach of this Canon, by whom would he be cut off?  I suppose by the Supreme Pontiff 364 (οἴομαι οὖν παρά τοῦ μείζονος ἀρχιερέως).

Footnotes

Can this mean the Pope?