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 CXIV

Canon CXIV.  (Greek cxv.)

That not only humble but also true is that voice of the Saints:  “If we say that we have no sin we deceive ourselves.”

It also seemed good that as St. John the Apostle says, “If we shall say that we have no sin we deceive ourselves and the truth is not in us,” whosoever thinks that this should be so understood as to mean that out of humility, we ought to say that we have sin, and not because it is really so, let him be anathema.  For the Apostle goes on to add, “But if we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all iniquity,” where it is sufficiently clear that this is said not only of humility but also truly.  For the Apostle might have said, “If we shall say we have no sins we shall extoll ourselves, and humility shall have no place in us;” but when he says, “we deceive ourselves and the truth is not in us” he sufficiently intimates that he who affirmed that he had no sin would speak not that which is true but that which is false.

Notes.

Ancient Epitome of Canon CXIV.

Whosoever shall interpret the saying of the Divine [i.e. St. John]:  “If we shall say that we have no sin, we deceive ourselves” as not being really true but as spoken out of humility, let him be anathema.

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