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 XII

[After Canon XX.]

Canon XII.  (Of the Latin Texts.)

Bishop Hosius said:  But some discretion is here requisite, brethren dearly beloved, in case some should come to those cities which are on the highway still ignorant of what has been decreed in the council.  The bishop of such a city ought therefore to admonish him [a bishop so arriving], and instruct him to send his deacon from that place.  Upon this admonition he must, however, himself return to his diocese.

Notes.

Van Espen.

This proposition of Hosius in the Roman Codex is joined as an appendix to the preceding canon.  The Greeks omit it altogether, very likely either because it seemed to be a proposition of Hosius’s rather than a synodal canon, for no adoption by the synod is recorded:  or else because, even if it were a decree, it was only of temporary character, that is to say, until the canons had been sufficiently promulgated, and therefore some on the ground of ignorance might be exempt from the threatened penalties.