The Canons of the Council in Trullo; Often Called The Quinisext Council 75Canon LXXV.

We will that those whose office it is to sing in the churches do not use undisciplined vociferations, nor force nature to shouting, nor adopt any of those modes which are incongruous and unsuitable for the church: but that they offer the psalmody to God, who is the observer of secrets, with great attention and compunction. For the Sacred Oracle taught that the Sons of Israel were to be pious.