I am reading the Sermons on Prayer by Saint Francis de Sales. They are short meditations on the goal, spirit and kinds of prayer. In his reflections on the importance of praying of the Divine Office for religious, he offers the following reflection:

Saint Augustine relates that once while he was still a pagan he entered a church where Saint Ambrose was having the Office chanted alternately [by two choirs], as it has been done since then. He was so enraptured and ecstatic that he thought he was in Paradise. Many persons assert that they have oftentimes seen troupe after troupe of angels coming to assist at the Divine Office. With what attention then ought we not to assist at it, seeing that the angels are present and repeat on high in the Church triumphant what we are saying here below.