Canon XV of the 1st-2nd Council of Constantinople (861) states that:

The rules laid down with reference to Presbyters and Bishops and Metropolitans are still more applicable to Patriarchs. So that in case any Presbyter or Bishop or Metropolitan dares to secede or apostasise from the communion of his own Patriarch, and fails to commemorate the latter’s name in accordance with custom duly fixed and ordained, in the divine Mystagogy, but, before a conciliar verdict has been pronounced and has passed judgement against him, creates a schism, the holy Council has decreed that this person shall be alienated from every priestly function if only he be convicted of having committed this transgression of the law. Accordingly, these rules have been sealed and ordained as respecting those persons who under the pretext of charges against their own presidents separate themselves, and create a schism and disrupt the union of the Church.

But as for those who on account of some heresy condemned by the Holy Councils, or Fathers, withdrawing themselves from communion with their president, who, that is to say, is preaching the heresy publicly, and teaching it barehead in church, such persons not only are not subject to any canonical penalty on account of their having walled themselves off from any and all communion with the one called a Bishop before any conciliar or synodal verdict has been rendered, but, on the contrary, they shall be deemed worthy to enjoy the honour which befits them among Orthodox Christians. For they have defied, not Bishops, but pseudo-bishops and pseudo-teachers; and they have not sundered the union of the Church with any schism, but, on the contrary, have been sedulous to rescue the Church from schisms and divisions.

Canon 15 of the 1st-2nd Council of Constantinople: Agapios and Nicodemus, The Rudder, p. 471