Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XV

Canon XV.

If any bishop, lying under any accusation, shall be judged by all the bishops in the province, and all shall unanimously deliver the same verdict concerning him, he shall not be again judged by others, but the unanimous sentence of the bishops of the province shall stand firm.

p. 116 Notes.

Ancient Epitome of Canon XV.

If all the bishops of a province agree with regard to a bishop already sentenced, a new trial shall not be granted him.

Van Espen.

By the phrase “by others” must be understood bishops called from a neighbouring province, of which mention is made in the previous canon, where in the case of an agreement among the bishops, the synod did not wish to be called in, even if it were demanded by the condemned bishop.  This canon, therefore, is a supplement as it were to the preceding.  And for this reason in the Breviarium and in Cresconius’s Collection of Canons they are placed under a common title, cap. 144, “Concerning the difference of opinion which happens in the judgment of bishops, or when a bishop is cut off by all the bishops of his province.”

From these canons it is manifest that at first the causes of bishops were agitated and decided in provincial synods, and this discipline continued for many centuries, and was little by little departed from in the VIIIth and IXth centuries.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa VI., Quæst. IV., Can. v.  Gratian adds a note which Van Espen remarks smacks of his own date rather than of that of the Synod of Antioch.