Matthew 6:5,6 (Luther)

Matthew 6:5. And when you pray, you must not be like the hypocrites; for they love to stand and pray in the synagogs and at the street corners, that they may be seen by men. Truly, I say to you, they have their reward.
6. 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.

In addition to almsgiving or doing good to our neighbor, praying is another work that is appropriate to the Christian. Just as the necessities of this life require us to do good to our neighbor and to sympathize with him in his need—after all, that is why we live together on earth, so that we might serve and help one another—so the constant threat to us in this life from every kind of inevitable danger and unavoidable need requires us to call upon God continually and to seek His help, both on our own behalf and on behalf of everyone else. But true almsgiving is a rare thing in this world. Not only is there so much ordinary robbing and stealing going on everywhere in the world that no one does anything good for his neighbor, but goes on scratching on his own manure pile without asking how his neighbor is getting along. But even if they do good works, they are only looking out for their own advantage. Thus the world is made up of nothing but robbers and thieves, both on the right and on the left, both physically and spiritually, both in bad works and in good works.
In the same way, praying is a rare work, which no one does but Christians. At the same time it was very common in the world. It was especially common among the Jews, as Christ shows here, in their synagogs and on street corners. And it is a common thing now in all these churches, monasteries, and convents. Day and night they are muttering and bawling with their singing and reading, and they fill the whole world with it. There is no shortage of praying; and yet if you put it all together, it is not worth a heller. Here Christ is denouncing and repudiating all their praying, in spite of the fact that they were practicing it so zealously, though only in order to make an impression on people and to get a reputation for piety. How much more damnable is the praying of our clergy! All they want to accomplish by it is to fill their own belly, and not one of them would say an Our Father if it did not bring him some money. Even when they have done their best, they have mumbled or chanted a sackful of words without feeling them or understanding them or believing them, as though they were bells or organs. In this way they have acquired a glorious reputation for being the only people who pray, while the other people are too occupied with secular affairs to be able to pray or serve God, so that they have to pray in our stead and we have to make lords of them by our money and goods.
Here we shall not go into the matter of how necessary prayer is, because of ourselves we ought to feel this. For we live in flesh and blood that is crammed full of wickedness of every sort. In addition, the world is always with us and against us, bringing all our misery and sorrow and trouble upon us. Besides, there is the devil everywhere around us, stirring up innumerable sects, factions, and corruptions, and driving us into unbelief and despair. Thus there is never any end to this, and there is no rest for us. We are surrounded by the kind of enemies who will not stop until they have knocked us down, and as individual poor men we are much too weak to withstand so many enemies. For that reason God says in the prophet Zechariah (Zech. 12:10) that He will give to those who are His own “the Spirit of grace and supplication” to preserve them while they are on the field of battle and to guard and protect them against that wicked and pernicious spirit. Therefore it is the particular work of Christians, who have the Spirit of God, not to be lax and lazy, but incessant and constant in their praying, as Christ teaches elsewhere (Luke 18:1).
But here the emphasis is on the fact that it must be a genuine prayer and not a piece of hypocrisy, as their prayers were and as our own used to be. Therefore, in instructing them how to pray correctly, Christ begins by showing them how they should go about it: they are not supposed to stand and pray publicly on the streets, but they should pray at home, in their own room, alone, in secret. This means that, above all, they should rid themselves of the false motive of praying for the sake of the appearance or reputation or anything of that sort. It does not mean that prayer on the street or in public is prohibited; for a Christian is not bound to any particular place and may pray anywhere, whether he is on the street or in the field or in church. All it means is that this must not be done out of regard for other people, as a means of getting glory or profit. In the same way He does not forbid the blowing of a trumpet or the ringing of a bell at almsgiving for its own sake, but He denounces the addition of a false motivation when He says: “in order to be seen by men.”
Nor is it a necessary part of this commandment that you have to go into a room and lock yourself in. Still, it is a good idea for a person to be alone when he intends to pray, so that he can pour out his prayer to God in a free and uninhibited manner, using words and gestures that he could not use if he were in human company. Although it is true that prayer can take place in the heart without any words or gestures, yet such things help in stirring up and enkindling the spirit even more; but in addition, the praying should continue in the heart almost without interruption. As we have said, a Christian always has the Spirit of supplication with him, and his heart is continually sending forth sighs and petitions to God, regardless of whether he happens to be eating or drinking or working. For his entire life is devoted to spreading the name of God, His glory, and His kingdom, so that whatever else he may do has to be subordinated to this.2
Nevertheless, I say, outward prayer must also go on, both individual prayer and corporate prayer. In the morning and in the evening, at table and whenever he has time, every individual should speak a benediction or the Our Father or the Creed or a psalm. And in assemblies the Word of God should be employed and thanks and petitions voiced to God for our general needs. This must necessarily be done in public, with a special time and place set aside for such assemblies. Such prayer is a precious thing and a powerful defense against the devil and his assaults. For in it, all Christendom combines its forces with one accord; and the harder it prays, the more effective it is and the sooner it is heard. At the present time, for example, it is of real benefit as a defense and a barrier against the many tricks which the devil might otherwise perpetrate through the members of his body. Thus it is certain that whatever still stands and endures, whether it is in the spiritual or in the secular realm, is being preserved through prayer.
But elsewhere I have often taken up and discussed the component parts and the characteristics which every real prayer has to possess,3 and therefore I shall only summarize them briefly here. They are as follows: first, the urging of God’s commandment, who has strictly required us to pray; second, His promise, in which He declares that He will hear us; third, an examination of our own need and misery, which burden lies so heavily on our shoulders that we have to carry it to God immediately and pour it out before Him, in accordance with His order and commandment; fourth, true faith, based on this word and promise of God, praying with the certainty and confidence that He will hear and help us—and all these things in the name of Christ, through whom our prayer is acceptable to the Father and for whose sake He gives us every grace and every good.
Christ indicates this by His use of one word when He says: “Pray to your Father who is in secret”; and later on He makes it even more explicit when He says: “Our Father who art in heaven.” For this is the same as teaching that our prayer should be addressed to God as our gracious and friendly father, not as a tyrant or an angry judge. Now, no one can do this unless he has a word of God which says that He wants to have us call Him “Father” and that as a father He has promised to hear us and help us. To do this, one must also have such a faith in his heart and a happy courage to call God his Father, praying on the basis of a hearty confidence, relying upon the certainty that the prayer will be heard, and then waiting for help.
But all these component parts were missing from the prayers of those Pharisees, who did not think beyond the question of how the work was to be done in order to give the impression that they were holy people who enjoyed praying; similarly, all that our monks and priests think of is how to use prayer as a means of filling their belly. So completely have they forgotten the necessity of this faith for proper prayer that it seems foolish or presumptuous to them for a person to claim with certainty that his prayer is pleasing to God and will be heard by Him. Although they kept on praying, therefore, they regarded it all as completely a risk; and thus they angered God terribly by their unbelief and their abuse of His name, contrary to both the First and the Second Commandment.
Learn, therefore, that there can be no real prayer without this faith. But do you feel weak and fearful? Your flesh and blood is always putting obstacles in the way of faith, as if you were not worthy enough or ready enough or earnest enough to pray. Or do you doubt that God has heard you, since you are a sinner? Then hold on to the Word and say: “Though I am sinful and unworthy, still I have the commandment of God, telling me to pray, and His promise that He will graciously hear me, not on account of my worthiness, but on account of the Lord Christ.” In this way you can chase away the thoughts and the doubts, and you can cheerfully kneel down to pray. You need not consider whether you are worthy or unworthy; all you need to consider is your need and His Word, on which He tells you to build. This is especially so because He has set before you the manner of praying and put into your mouth the words you are to use when you pray, as follows here. Thus you may joyfully send up these prayers through Him and put them into His bosom, so that through His own merit He may bring them before the Father.
Luther, M. (1999, c1956). Vol. 21: Luther's works, vol. 21 : The Sermon on the Mount and the Magnificat (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (21:137). Saint Louis: Concordia Publishing House.