There is another inward kind of prayer without ceasing, which is the desire of the heart.

St. Augustine of Hippo

I have roared with the groaning of my heart. There is a secret groaning, which is not heard by man: yet if the thought of some strong desire has taken so strong hold of the heart, that the wound of the inner man finds expression in some uttered exclamation, the reason of it is asked; and a man says to himself, Perhaps this is the cause of his groaning;and, Perhaps this or that has befallen him. Who can determine, but He in whose Eyes and Ears he groaned?

Therefore he says, I roared with the groaning of mine heart; because if men ever hear a man’s groanings, they for the most part hear but the groaning of the flesh; they do not hear him who groans with the groaning of his heart. Some one has carried off his goods; he roars, but not with the groaning of his heart: another because he has buried his son, another his wife; another because his vineyard has been injured by a hailstorm; another because his cask has turned sour; another because some one has stolen his beast; another because he has suffered some loss; another because he fears some man who is his enemy: all these roar with the groaning of the flesh.

The servant of God, however, because he roars from the recollection of the Sabbath, where the Kingdom of God is, which flesh and blood shall not possess, says, I have roared with the groaning of my heart.

And who observed and noticed the cause of his groaning? All my desire is before You (Psalm 37:9). For it is not before men who cannot see the heart, but it is before You that all my desire is open! Let your desire be before Him; and the Father, who sees in secret, shall reward you. (Matthew 6:6)

For it is your heart’s desire that is your prayer; and if your desire continues uninterrupted, your prayer continues also. For not without a meaning did the Apostle say, Pray without ceasing.(1 Thessalonians 5:17) Are we to be without ceasing bending the knee, prostrating the body, or lifting up our hands, that he says, Pray without ceasing? Or if it is in this sense that we say that we pray, this, I believe, we cannot do without ceasing.

There is another inward kind of prayer without ceasing, which is the desire of the heart. Whatever else you are doing, if you do but long for that Sabbath, you do not cease to pray. If you would never cease to pray, never cease to long after it. The continuance of your longing is the continuance of your prayer.

You will be ceasing to speak, if you cease to long for it. Who are those who have ceased to speak? They of whom it is said, Because iniquity shall abound, the love of many shall wax cold. (Matthew 24:12) The freezing of charity is the silence of the heart; the burning of charity is the cry of the heart.

Therefore follows, And my groaning is not hid from You. From You indeed it is not hid; but from many men it is hid. The servant of God sometimes seems to be saying in humility, And my groaning is not hid from You. Sometimes also he seems to smile. Is then that longing dead in his heart? If however there is the desire within, there is the groaning also. It does not always find its way to the ears of man; but it never ceases to sound in the ears of God.

Source: Commentary on the Psalms (In ps. 37, 13-14; CCL 38, 391-2)