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 XIX

Canon XIX.

If any of the ascetics, without bodily necessity, shall behave with insolence and disregard the fasts commonly prescribed and observed by the Church, because of his perfect understanding in the matter, let him be anathema.

Notes.

Ancient Epitome of Canon XIX.

Whoso neglects the fasts of the Church, let him be anathema.

I have followed Hefele’s translation of the last clause, with which Van Espen seems to agree, as well as Zonaras.  But Hardouin and Mansi take an entirely different view and translate “if the Eustathian deliberately rejects the Church fasts.”  Zonoras and Balsamon both refer to the LXIXth of the Apostolical Canons as being the law the Eustathians violated.  Balsamon suggests that the Eustathians shared the error of the Bogomiles on the subject of fasting, but I see no reason to think that this was the case; Eustathius’s action seems rather to be attributable to pride, and a desire to be different and original, “I thank thee that I am not as other men are,” (as Van Espen points out).  All that Socrates says (H. E. II., xliii.) is that “he commanded that the prescribed fasts should be neglected, and that the Lord’s days should be fasted.”

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. xxx., c. viii., in an imperfect translation but not that of either Isidore or Dionysius.