John Cassian, The Institutes II.8: Of the Prayer which follows the Psalm

That practice too which we have observed in this country—viz., that while one sings to the end of the Psalm, all standing up sing together with a loud voice, “Glory be to the Father and to the Son and to the Holy Ghost”—we have never heard anywhere throughout the East, but there, while all keep silence when the Psalm is finished, the prayer that follows is offered up by the singer. But with this hymn in honour of the Trinity only the whole Psalmody is usually ended.