No one shall join in prayers with heretics or schismatics.
Ancient Epitome of Canon XXXIII.
Thou shalt not pray with heretics or schismatics.
The underlying principle of this canon is the same as the last, for as the receiving of p. 150 the Eulogiæ which were sent by heretics as a sign of communion, signified a communion with them in religious matters, so the sharing with them common prayer is a declaration of the same communion, and therefore to be avoided. This is also set forth in Apostolical Canon number xlv.