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 XXI

Canon XXI.  (Greek xxiv.)

That the sons of clergymen are not to be joined in marriage with heretics.

Likewise it seemed good that the sons of clergymen should not be joined in matrimony with gentiles and heretics.

p. 453 Notes.

Ancient Epitome of Canon XXI.

[The same as the canon.]

This is Canon xij. of Hippo, 393.