From: Matthew 6:1-6, 16-18
An Upright Intention in Almsgiving, Prayer and Fasting
(Jesus said to His disciples,)  "Beware of practising your piety before men in order to be seen by them; for then you will have no reward from your Father who is in Heaven.
 "Thus, when you give alms, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by men. Truly, I say to you, they have their reward.  But when you give alms, do not let your left hand know what your right hand is doing,  so that your alms may be in secret; and your Father who sees in secret will reward you.
 "And when you pray, you must not be like the hypocrites; for they love to stand and pray in the synagogues and at the street corners, that they may be seen by men. Truly, I say to you, they have their reward.  But when you pray, go into your room and shut the door and pray to your Father who is in secret; and your Father who sees in secret will reward you.
 "And when you fast, do not look dismal, like the hypocrites, for they disfigure their faces that their fasting may be seen by men. Truly, I say to you, they have their reward.  But when you fast, anoint your head and wash your face,  that your fasting may not be seen by men but by your Father who is in secret; and your Father who sees in secret will reward you."
1-18. "Piety", here, means good works (cf. note on Matthew 5:6). Our Lord is indicating the kind of spirit in which we should do acts of personal piety. Almsgiving, fasting and prayer were the basic forms taken by personal piety among the chosen people--which is why Jesus refers to these three subjects. With complete authority He teaches that true piety must be practised with an upright intention, in the presence of God and without any ostentation. Piety practised in this way implies exercising our faith in God who sees us--and also in the safe knowledge that He will reward those who are sincerely devout.
5-6. Following the teaching of Jesus, the Church has always taught us to pray even when we were infants. By saying "you" (singular) our Lord is stating quite unequivocally the need for personal prayer--relating as child to Father, alone with God.
Public prayer, for which Christ's faithful assemble together, is something necessary and holy; but it should never displace obedience to this clear commandment of our Lord: "When you pray, go into your room and shut the door and pray to your Father".
The Second Vatican Council reminds us of the teaching and practice of the Church in its liturgy, which is "the summit toward which the activity of the Church is directed; it is also the fount from which all her power flows [...]. The spiritual life, however, is not limited solely to participation in the liturgy. The Christian is indeed called to pray with others, but he must also enter into his bedroom to pray to his Father in secret; furthermore, according to the teaching of the Apostle, he must pray without ceasing (1 Thessalonians 5:17)" ("Sacrosanctum Concilium", 10 and 12).
A soul who really puts his Christian faith into practice realizes that he needs frequently to get away and pray alone to his Father, God. Jesus, who gives us this teaching about prayer, practised it during His own life on earth: the holy Gospel reports that He often went apart to pray on His own: "At times He spent the whole night in an intimate conversation with His Father. The Apostles were filled with love when they saw Christ pray" ([St] J. Escriva, "Christ Is Passing By", 119; cf. Matthew 14:23; Mark 1:35; Luke 5:16; etc.). The Apostles followed the Master's example, and so we see Peter going up to the rooftop of the house to pray in private, and receiving a revelation (cf. Acts 10:9-16). "Our life of prayer should also be based on some moments that are dedicated exclusively to our conversation with God, moments of silent dialogue" ("ibid", 119).
16-18. Starting from the traditional practice of fasting, our Lord tells us the spirit in which we should exercise mortification of our senses: we should do so without ostentation, avoiding praise, discreetly; that way Jesus' words will not apply to us: "they have their reward"; it would have been a very bad deal. "The world admires only spectacular sacrifice, because it does not realize the value of sacrifice that is hidden and silent" ([St] J. Escriva, "The Way", 185).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.