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 CXVII

Canon CXVII.  (Greek cxviii.)

Of peoples converted from the Donatists.

Item, it seemed good, since it was so decreed some years ago by a plenary council, that whatever churches were erected in a diocese before the laws were made concerning Donatists when they became Catholic, should pertain to the sees of those bishops through whom their return to Catholic unity was brought about; but after the laws whatever churches communicated were to belong there where they belonged when they were Donatists.  But because many controversies afterward arose and are still springing up between bishops concerning dioceses, which were not then at all in prospect, now it has seemed good to this council, that wherever there was a Catholic and a Donatist party, pertaining to different sees, at whatever time unity has been or shall be made, whether before or after the laws, the churches shall belong to that see to which the Catholic church which was already there belonged.

Notes.

Ancient Epitome of Canon CXVII.

Whenever conversions and unions of Donatists are effected, let them be subject to that throne to which the Catholic Church which was formerly there was subject.

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