The disciple: What is the acme of all labours of asceticism, which a man when he has reached it, recognises as the summit of his course?
The teacher: When he is deemed worthy of constant prayer. When he has reached this, he has touched the end of all virtues and forthwith he has a spiritual dwelling place. If a man has not received in truth the gift of the Comforter, it is not possible for him to accomplish constant prayer in quiet. When the spirit takes its dwelling place in a man he does not cease to pray because the spirit will constantly pray in him. Then, neither when he sleeps, nor when he is awake, will prayer be cut off from his soul; but when he eats and when he drinks, when he lies down or when he does any work, even when he is immersed in sleep, the perfumes of prayer will breathe in his soul spontaneously. And henceforth, he will not possess prayer at limited times but always; and when he has outward rest, even then prayer is ministered unto him secretly. For the silence of the serene is prayer, says a man clad with Christ. For their deliberations are divine impulses. The motions of the pure mind are quiet voices with which they secretly chant psalms to the Invisible One.
[Mar Isaac the Syrian, Ascetical Homilies, XXXV]