Canon 13
On the prohibition of clergy [and readers] serving in another city without “representative” letters.
Foreign clergy and readers in another city should not serve in any way anywhere without representative letters from their bishop.
Greek original
Ξένους κληρικοὺς καὶ ἀγνώστους, ἐν ἑτέρᾳ πόλει, δίχα συστατικῶν γραμμάτων τοῦ ἰδίου ἐπισκόπου, μηδόλως μηδαμοῦ λειτουργεῖν.
Parallel canons
Ап.12133233IVВс.11Трул.17