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 XII

Canon XII.

If any bishop out of Synod time shall have fallen under accusation, let his cause be heard by 12 bishops.

Felix the bishop, said:  I suggest, according to the statutes of the ancient councils, that if any bishop (which may God forbid!) shall have fallen under any accusation, and there shall have been too great necessity to wait for the summoning of a majority of the bishops, that he may not rest under accusation, let his cause be heard by 12 bishops; and let a presbyter be heard by six bishops with his own bishop, and a deacon shall be heard by three.

Notes.

Ancient Epitome of Canon XII.

When a bishop is to be tried, if the whole synod does not sit, let at least twelve bishops take up the matter; and for the case of a presbyter, six and his own diocesan; and for the case of a deacon, three.

This is Canon x. of the Synod of Genethlius.

Johnson.

Hereby must be meant African canons; that under Gratus [a.d. 348] had decreed the same thing.

Who was the bishop’s judge at the first instance does not appear by this canon; but it is natural to suppose it was the Primate.  It is probable that this canon is to be understood of hearing upon an appeal, because it is certain that a priest’s cause, at the first instance, was to be tried before the bishop (see Can. 10, 11).  And therefore the latter part of the canon can be understood of no hearing but by way of appeal, nor by consequence the former.  And this seems more clear by Can. Afr. 29.