On the Outward Forms of Prayer

Gen. 17:3 Then Abram fell on his face, and God talked with him…
Num. 20:6 So Moses and Aaron went from the presence of the assembly to the door of the tabernacle of meeting, and they fell on their faces. And the glory of the LORD appeared to them.
Jos. 7:6 Then Joshua tore his clothes, and fell to the earth on his face before the ark of the LORD until evening, he and the elders of Israel; and they put dust on their heads.
1 Kgs. 18:42 And Elijah went up to the top of Carmel; then he bowed down on the ground, and put his face between his knees
Neh. 8:6 And Ezra blessed the LORD, the great God. Then all the people answered, “Amen, Amen!” while lifting up their hands. And they bowed their heads and worshiped the LORD with [their] faces to the ground.
Ps. 63:4 Thus will I bless thee while I live: I will lift up my hands in thy name.
Ps. 95:6 Oh come, let us worship and bow down; Let us kneel before the LORD our Maker.
Ps. 135:2 Praise the LORD! Praise the name of the LORD; Praise [Him], O you servants of the LORD! You who stand in the house of the LORD, In the courts of the house of our God…
Mat. 17:5-6 While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, “This is My beloved Son, in whom I am well pleased. Hear Him!” And when the disciples heard it, they fell on their faces and were greatly afraid.
Mat. 26:39 He went a little farther and fell on His face, and prayed, saying, “O My Father, if it is possible, let this cup pass from Me; nevertheless, not as I will, but as You [will].”
Mar. 11:25 And whenever you stand praying, forgive, if you have anything against anyone, so that your Father also who is in heaven may forgive you your trespasses…
Eph. 3:14 For this reason I bow my knees to the Father of our Lord Jesus Christ
1 Tim. 2:8 I will therefore that men pray every where, lifting up holy hands, without wrath and doubting.
Rev. 7:11 All the angels stood around the throne and the elders and the four living creatures, and fell on their faces before the throne and worshiped God…
St. Isaac of Syria died ca. 700
If someone decides to abandon what belongs in the first place, without having found what comes afterwards, then it is clear that he is being mocked by demons…It is in proportion to the honour which someone shows in his person to God during prayer, both with his body and mind, that the door of assisitance wil be opened for him, leading to the purifying of the impulses and to illumination in prayer. Someone who shows a reverential posture during prayer, by stretching out his hands to heaven as he stands in modesty, or by falling on his face to the ground, will be counted worthy of great grace from on high. Anyone who continuously adorns his prayer with such outward postures will swiftly and quickly be accounted worthy of the activinty of the Holy Spirit, for the Lord is accounted great in his eyes, thanks to the honour he shows in the sacrifices which he presents before the Lord at those times which have been set apart for him by the law of freewill.
You should realize, my brothers, that in all our service God very much wants outward postures, specific kinds of honour, and visible forms of prayer – not for His own sake, but for our benefit.  He Himself is not profited by such things, nor does He lose anything when they are neglected; rather, they are for the sake of our feeble nature. Had such things not been requisite, He Himself would not have adopted such outward postures during His Incarnation – thus speaking with us in the Holy Scriptures. He cannot be dishonoured by anything, seeing honour belongs to Him by His very nature. But we, as a result of slovenly habits and various outward actions which lack reverence, have acquired an attitude of mind that shows contempt towards Him. Consequently we fall from grace of our own volition, seeing that we are subject to backsliding; then we are assailed by incessant attacks and continual deception from the demons, as we acquire a nature that loves comfort and is easily swayed to evil actions.
Many people have depised these outward postures in their thoughts and supposed that prayer of the heart suffices by itself for God, claiming, as they lie on their backs or  are sitting in a disrespectful manner, that there should only be an interior recollection of God; they are not concerned at all with adorning the visible side of their worship with prolonged standing, corresponding to their body’s strength, or with making the venerable sign of the cross over their organs of the senses. Nor are they concerned, as they kneel on the ground, to act like those about to draw near to a flame and themselves assume, both inwardly and outwardly, a reverential posture, or to accord special honour to the Lord, honouring Him with all their limbs and with reverence on their faces.This is because they have not perceived the might of the adversary they have, and as a result they are handed over to the workings of falsehood, not having understood that they are still mortal and liable to be stirred by their soul, which is subject to backsliding; they do not realize that they have not reached the state of spiritual beings, or that the resurrection has not yet taken place and they have not yet achieved a state of immutibility. During the body’s life, human nature is in need of labour and training in new things all the time, they have wanted to lead their lives in a purely spiritual state, without being involved in those things which necessarily constrain the world which is subject to the passions.
…[E]rror came when certain people abandoned prayer’s venerable outward forms, turning instead to thier own rules and special customs which they had laid down for themselves according to their own whim, and when they completely deprived themselves of the Holy Mysteries, instead despising and scorning them; when they deprived themselves furthermore from the light of the divine Scriptures and failed to study the teaching of the words of the Fathers which give instructions about the stratagems against the demons; and when they gave up the various acts of lowliness, prostrations, continual falling on the ground, a suffering heart and the submissive postures appropiate to prayer, modest hands standing, hands clasped in submissive fashion, or stretched out to heaven, the senses respectful during prayer. Instead, they seized upon various forms of pride, as a result mingling their prayer with insult towards God. They accompanied their prayer with haughty outward postures, forgetting how exalted is the Divine Nature and how their own nature is but dust. Yet in all this the words of their prayer were no different from those of the psalms. (The Spiritual World of St. Isaac the Syrian by Hilarion Alfeyev, pp. 154-158)