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 CXXXII

Canon CXXXII.  (Greek cxxxiii.)

Concerning a bishop who removes a man from communion who says he has confessed to the bishop alone his crime.

It also seemed good that if on any occasion a bishop said that someone had confessed to him alone a personal crime, and that the man now denies it; let not the bishop think that any slight is laid upon him if he is not believed on his own word alone, although he says he is not willing to communicate with the man so denying through a scruple of his own conscience.

Notes.

Ancient Epitome of Canon CXXXII.

If a bishop says “someone has confessed to me alone a crime,” if the someone denies it, he [i.e. the bishop] is not easily to be believed.

N.B.  The word used for “someone” in the Epitome is πέλας, which ordinarily means a “neighbour” but may mean “any one.”  Vide Liddell and Scott.