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 XXXIV

Canon XXXIV.

No Christian shall forsake the martyrs of Christ, and turn to false martyrs, that is, to those of the heretics, or those who formerly were heretics; for they are aliens from God.  Let those, therefore, who go after them, be anathema.

Notes.

Ancient Epitome of Canon XXXIV.

Whoso honours an heretical pseudo-martyr let him be anathema.

Hefele.

This canon forbids the honouring of martyrs not belonging to the orthodox church.  The number of Montanist martyrs of Phrygia was probably the occasion of this canon.

The phrase which I have translated “to those who formerly were heretics” has caused great difficulty to all translators and scarcely two agree.  Hammond reads “those who have been reputed to have been heretics;” and with him Fulton agrees, but wrongly (as I think) by omitting the “to.”  Lambert translates “to those who before were heretics” and correctly.  With him agrees Van Espen, thus, vel eos qui prius heretici fuere.