In your longing, in your appeal, is your prayer, says Saint Augustine
'Lord all my longing is known to thee' (Psalm 36: 9). It is not known to men, who are incapable of seeing the heart, but to thee is all my longing known.
Let your longing be known to God, 'and your Father who sees in secret will reward you' (Matthew.6:6). There is your prayer, in your longing, and if your longing is continual, so also is your prayer.
And so, it is not in vain that the Apostle says to us:
'Pray constantly' (1 Thessalonians 5:17).
But shall we always have bended knees, prostrated bodies and uplifted hands, because Saint Paul says to us: 'Pray constantly'? If that is what we mean by prayer, I do not think that we shall be able to engage in it without interruption. There is another prayer, interior and uninterrupted: our longing. Whatever you may be doing, if you long for the peace of heaven, you never cease to pray. That is why, if you do not wish to cease from praying, you do not cease from longing. Your constant longing will be like an uninterrupted appeal. And if you cease to love, you will become dumb. Who are they, the dumb? Those of whom it is said: 'Because wickedness is multiplied, most men's love will grow cold (Matthew 24:12). Dumbness of heart is love grown cold; the cry of the heart is love on fire. If your love remains, you cry out unceasingly; if you cry out unceasingly, your longing never ceases; if you long, you remember the peace of heaven, and it is important that you should understand to whom it is that the sighing of your hearts goes up.
'All my longing is known to thee'. And if the longing is known to the Lord, is not the sighing known too? It must be, because sighing is nothing but the expression of longing. And so the psalmist continues: 'And my sighing is not hidden from thee'. It is not hidden from God, but it is hidden from the mass of mankind. Thus it is that we sometimes hear a humble servant of God exclaim: 'My sighing is not hidden from thee', even while that servant of God looks thoroughly cheerful. Would longing for that reason be dead in his heart? Longing is there, and sighing as well. Even if it does not reach the ears of men, it is never far from the ear of God.
From Saint Augustine's Commentary on Psalm 36 (37)