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

Canon XXXVII.

Since at different times there have been invasions of barbarians, and therefore very many cities have been subjected to the infidels, so that the bishop of a city may not be able, after he has been ordained, to take possession of his see, and to be settled in it in sacerdotal order, and so to perform and manage for it the ordinations and all things which by custom appertain to the bishop:  we, preserving honour and veneration for the priesthood, and in no wise wishing to employ the Gentile injury to the ruin of ecclesiastical rights, have decreed that those who have been ordained thus, and on account of the aforesaid cause have not been settled in their sees, without any prejudice from this thing may be kept [in good standing] and that they may canonically perform the ordination of the different clerics and use the authority of their office according to the defined limits, and that whatever administration proceeds from them may be valid and legitimate.  For the exercise of his office shall not be circumscribed by a season of necessity when the exact observance of law is circumscribed.

p. 383 Notes.

Ancient Epitome of Canon XXXVII.

A bishop who, on account of the incursions of the barbarians, is not set in his throne, shall have his own chair of state, and shall ordain, and shall enjoy most firmly all the rights of the priesthood.

By Canon XVIII. of Antioch the principle of this canon was enunciated, that when a bishop did not take possession of his see because he could not do so, he was not to be held responsible or to lose any of his episcopal rights and powers, in that case the impossibility arose from the insubordination of the people, in this from the diocese being in the hands of the barbarians.

It has been commonly thought that the Bishops in partibus infidelium had their origin in the state of things calling for this canon.