Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XLII

Canon XLII.

None of the priesthood nor of the clergy may travel without letters canonical.

Notes.

Ancient Epitome of Canons XLI. and XLII.

No clergyman shall undertake a journey without canonical letters or unless he is ordered to do so.

Van Espen.

(On Canon xli.)

It is well known that according to the true discipline of the Church no one should be ordained unless he be attached to some church, which as an ecclesiastical soldier he shall fight for and preserve.  As, then, a secular soldier cannot without his prefect’s bidding leave his post and go to another, so the canons decree that no one in the ranks of the ecclesiastical military can travel about except at the bidding of the bishop who is in command of the army.  A slight trace of this discipline is observed even to-day in the fact that priests of other dioceses are not allowed to celebrate unless they are provided with Canonical letters or testimonials from their own bishops.

(On Canon xlii.)

The whole subject of Commendatory and other letters is treated of in the note to Canon VIII. of the Council of Antioch.

Canon xlj. is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars III., Dist. V., De Consecrat, can. xxxvj.

Canon xlij. is appended to the preceding, but, curiously enough, limited to laymen, reading as follows:  “a layman also without canonical letters,” that is “formed letters,” should not travel anywhere.  The Roman Correctors remark that in the Greek order this last is canon xli., and the former part of Gratian’s canon, canon xlij. of the Greek, but such is not the order of the Greek in Zonaras nor in Balsamon.  The correctors add that in neither canon is there any mention made of laymen, nor in Dionysius’s version; the Prisca, however, read for canon xlj., “It is not right for a minister of the altar, even for a layman, to travel, etc.”