When the Spirit dwells within a person, from the moment that person has become prayer, the Spirit never leaves them. For the Spirit himself never ceases to pray within us. Whether we are asleep or awake, from then on prayer never departs from our soul. Whether we are eating or drinking or sleeping or whatever else we may be doing, even if we are in the deepest of sleeps, the incense of prayer is rising without effort in our heart. Prayer never again deserts us. In every moment of our life, even when it appears to have ceased, prayer is secretly at work within us continuously.
One of the Fathers, the bearers of Christ, teaches that prayer is the silence of the pure in heart; for their very thoughts are the movements of God. The movements of the heart and the intellect that have been purified become voices full of sweetness with which such people never cease to sing in secret to the hidden God.
— From The Ascetical Treatises of Isaac of Nineveh (613-700)