The Council in Trullo (Quinisext Council, 692 AD)

Since clerics of different churches have left their own churches in which they were ordained and betaken themselves to other bishops, and without the consent of their own bishop have been settled in other churches, and thus they have proved themselves to be insolent and disobedient; we decree that from the month of January of the past IVth Indiction no cleric, of whatsoever grade he be, shall have power, without letters dimissory of his own bishop, to be registered in the clergy list of another church. Whoever in future shall not have observed this rule, but shall have brought disgrace upon himself as well as on the bishop who ordained him, let him be deposed together with him who also received him.

Ancient Epitome of Canon XVII.

Whoever receives and ordains a wandering cleric shall be deposed together with him thus wickedly ordained.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa XXI., Quæst., ii. can. j.