7 Be regular (instate) in prayers at the appointed hours and times. In the oratory let no one do anything else than the duty for which the place was made, and from which it has received its name; so that if any of you, having leisure, wish to pray at other hours than those appointed, they may not be hindered by others using the place for any other purpose. In the psalms and hymns used in your prayers to God, let that be pondered in the heart which is uttered by the voice; chant nothing but what you find prescribed to be chanted; whatever is not so prescribed is not to be chanted.
Source: Letters (New Advent)