The Lord completes this triad of hidden righteousness by correcting the practice of fasting. As almsgiving and prayer can be corrupted by the desire of human praise, so too can fasting, which is meant to humble the soul. Those who make a display of their abstinence do not mortify pride, but nourish it; and thus even self-denial becomes an occasion for vanity.

The sadness and disfigurement of the hypocrite are not signs of devotion, but of self-advertisement. They seek to be known as fasting rather than to be healed by it. Having desired the notice of men, they obtain it, and with it the whole of their reward.

By contrast, the command to anoint the head and wash the face signifies a disposition of inward joy and composure. The fasting that pleases God does not parade affliction, but preserves serenity, so that the body’s restraint does not proclaim itself outwardly. Thus the soul learns to discipline itself without drawing attention to its discipline.

Fasting is therefore ordered, not to the opinion of others, but to the sight of God alone. What is hidden from men is seen by Him who searches the heart. And the reward promised is not human admiration, but the strengthening of the interior man, who learns to prefer God to bodily satisfaction.

In this way the Lord binds together mercy, prayer, and self-denial under one rule: righteousness must be practiced for God, not for display. When these works are done in secret, they purify the heart; when they are done for praise, they lose their power. Thus the soul is trained to seek its joy where its reward truly lies.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 7-8