Canons of Sardica, Carthage, and Related Synods

Canon CXIX. (Greek cxx.)

That if a bishop shall possess a diocese which he has snatched from heresy for three years, no one may take it from him.

Item, it seemed good that if anyone after the laws should convert any place to Catholic unity and retain it for three years without opposition, it should not be taken away from him afterwards. If however there was during those three years a bishop who could claim it and was silent, he shall lose the opportunity. But if there was no bishop, no prejudice shall happen to the see, but it shall be lawful when the place that had none shall receive a bishop, to make the claim within three years of that day. Item, if a Donatist bishop shall be converted to the Catholic party, the time that has elapsed shall not count against him, but from the day of his conversion for three years he shall have the right of making a claim on the places which belonged to his See.

Ancient Epitome of Canon CXIX.

Whosoever shall convert a region to Orthodoxy and shall keep it converted for three years, let him be without blame. But if the bishop converted from Donatus within three years of its conversion seeks his diocese again, let it be returned to him (εἰ ἐνάγει, ἐναγέτω.)

This is Canon xj. of Carthage, a.d. 418.