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 CXXVIII

Canon CXXVIII.  (Greek cxxix.)

That those out of communion should not be allowed to bring accusation.

It seemed good to all, as it had been decreed by the former councils, concerning what persons were to be admitted to bring accusations against clerics; and since it had not been expressed what persons should not be admitted, therefore we define, that he cannot properly be admitted to bring an accusation, who had been already excommunicated, and was still lying under that censure, whether he that wished to be the accuser were cleric or layman.

Notes.

Ancient Epitome OF Canon CXXVIII.

One excommunicated is not to give witness.

The Council of Carthage of 419 had at its first session on May 25th done thus much.  But when it met again on the 30th of the same month, it continued the code.  The introduction in regard to this new session is this introduction.  The Canons then enacted were original, viz. numbers 128, 129, 130, 131, 132 and 133.