Imam ar-Rida (AS) is quoted to have said: " The servant has been commanded to perform the wudu' (ritual ablution) so as to be pure when standing before the All-Powerful and supplicating, and by obeying Him, to be purged from filth and impurity, beside his removing laziness, expelling sleep and purifying the heart to stand in the Presence of the All-Powerful. Confining it (the wudu') only to the face, the two hands, the head and the two feet, was because when the servant stands before the All-Powerful, the parts which are exposed are those which are ordered to be washed in the wudu': as with his face he performs the sujud (prostration), with his hands he requests, desires, dreads and supplicates, with his head he inclines to Him in his ruku. ' (bowing down) and his sujud, and with his legs he stands and sits..."  etc. Up to here he explained the principal point in the wudu " informing the people of knowledge and suluk that to stand in the holy Presence of Allah, the of worship, and of obeying the Lord. Therefore, the purification of the outside results in the purification of the inside, and purging the exterior leads to the purity of the interior. Generally, the traveler to Allah must, at the time of wudu' be aware of his being about to stand in the Presence of Allah, the Almighty, as with such states of heart as he has, he does not Glorified and Most High, and to offer supplication to the Provider of Needs, require certain disciplines which should be observed. One must not appear in His Presence even with the external filths and impurities and with a sleepy eye, let alone with a heart which is filled with dirts, and it is afflicted with spiritual impurities, which are the origin of all impurities. Despite the fact that a narrative says: "Allah, the Exalted, does not look at your faces, but He looks at your hearts" , and despite the fact that the means with which man attends to Allah, the Exalted, and what is, in the worlds of creation, worthy of looking at His Majesty, Greatness and Glory, is the heart, while the other organs have no share in it, yet, they did not neglect the outer cleanliness. So, they decided the external purification for cleaning man's exterior, and the inner purification for cleaning his interior. In this noble hadith , it is clear from assigning the purification of the heart to be a result of the wudu' that the wudu' has an interior with which man's interior is purified, and meanwhile it appears that there is a connection between the exterior and the interior, the visible and the invisible. Similarly it becomes clear that the outer cleanliness, the outside wudu', is of the acts deserve to be in His Presence, or he may even be dismissed from the Presence of the Lord, the Most High. Thus, he has to get ready to have his outer purification transferred to his interior, and to purify his heart -which is the object of Allah's attention, or, actually, is the lodging of His Sanctity -from all that is other than Allah, and to take out from his head any notion of arrogance and I-ness, which is the origin of the origins of the impurities, so as to become worthy of His Presence. After that, Imam ar-Rida (AS) explains the reason for specifying certain organs to wud'u'. He says:
"Confining it only to the face, the two hands, the head and the two feet, was because when the servant stands before the All-Powerful, the parts which are exposed are those which are ordered to be washed in the wudu': as with his face he performs the sujud (prostration), with his hands he requests, desires, dreads and supplicates, with his head he inclines to Him in his ruku' (bowing down) and his sujud, and with his legs he stands and sits..."
The gist of his discourse is that these organs take part in worshipping Allah, and it is through these organs that worship is manifested. Consequently, it is necessary to purify them. Then he refers to those acts of worship which appear from them, opening the way of their being valid and useful to the deserving people, and making the people of knowledge familiar with these secrets that the organs on which servitude appears in Allah's Blessed Presence, should be clean and purified, as the outer limbs and organs of the body, which have a deficient share of those meanings, would not be worthy of that station without purification. Although submission is not, actually, a character of the face, and none of requesting, desiring, dreading, supplicating and facing the qiblah belongs to any of the tangible organs, yet, as these organs are the manifestations of those meanings, they must be purified. Therefore, purifying the heart, which is the real place of servitude, and the actual centre of those meanings, is more necessary. Without its purification, the external organs will never be purified even if they are washed in seven seas, and it (the heart) will not deserve being in the Presence of Allah. Actually, Satan will have a hand in it, and it will be dismissed from His Glorious Presence.
Connection: In an authorized narrative in 'Ilalush-Shara'i' it is related that: "A group of Jews came to the Messenger of Allah (SA) and asked him questions. Among their questions they asked: "Tell us, 0 Muhammad why, are these four organs given the wudu', while they are the cleanest parts of the body'?" The Messenger of Allah (SA) said: 'When Satan whispered to Adam (AS) and he came near the "tree" and looked at it, he lost face. He stood up and walked to it -the first step taken towards sinning. He took with his hand some of what was of it and ate it. Off his body flew what were on him of jewelry and apparel. He put his hand on the top of his head and wept. Allah accepted his repentance, but made it incumbent upon him and his offspring to purify those four organs. So, Allah ordered the face to be washed, because it looked at the "tree". He ordered the hands to be washed to the elbows, because he took with them (the fruit of the tree). He ordered the head to be anointed (with the hand wet with water), as he put his hand on the top of his head, and He ordered the feet to be anointed because with them he walked to sin." 
Concerning the reason for imposing fasting, there is also a noble hadith to the effect that the Jews asked him: "What caused Allah to impose on your people to fast for thirty days?" He said: "It was Adam (AS), because that which he had eaten from that "tree" remained in his stomach for thirty days. So Allah made it incumbent upon him and his offspring to endure hunger and thirst for thirty days ,and He allowed them, out of His kindness, to eat and drink at the nights." 
These noble hadiths give the people of allusion and the people of heart to understand many points: Although Adam's sin was not like the sins of the others, as it might have been a natural one, or a sin of being inclined to multiplicity, the tree of nature, or of attending to multiplicity of names after the attraction [jadhibah] of self-annihilation [fana'-i dhati], yet it was not expected from one like Adam (AS) who was Allah's chosen one [safiy] and distinguished by proximity [qurb] and self-annihilation. Therefore, according to the love-zeal [ghairat-i hubbi] of His Sanctified Essence, He announced his disobedience and going astray to all the worlds and on the tongues of all the prophets (AS). He, the Exalted, said: "And Adam disobeyed his Lord, so he went astray."  Thus, so much cleaning and purification were needed for him (Adam) and his offspring who were hidden in his loin and (so) participated in the sin, though they did participate (in it) after coming out of the loin, too.
Therefore ,the sin committed by Adam and his offspring has many degrees and manifestations. The first of those degrees is paying attention to multiplicity of names, and the last of those manifestations is eating from the forbidden tree, the invisible [malakuti] form of which is a tree that carries diverse sorts of fruits. And its visible [mulki] form is its nature and affairs, and the love of this world and the self, as seen now in his offspring, is of the affairs of the same inclination to that tree and eating from it. Similarly, for their cleaning, purifying, cleansing, salat and fasting for the redemption of the father's sin which is the origin -there are many degrees in proportion to the degrees of the sin. From this explanation it is understood that all kinds of disobedience of the children of Adam are related to eating from the "tree", and are purified in a certain way. All their sins of the heart are also related to that tree, and are purified in a certain way. Then, all kinds of spiritual sins are related to it, too, and are purified in a certain way.
Purifying the external organs is the "shadow" [zill] of the purity of the heart and spirit for the perfect. It is an order, and a "means" to them, for the people of suluk. As long as man is within the veil of the individuation [ta'ayyun] of the organs and their purification, and he lingers there, he cannot be of the people of the suluk, and is still in the sin. But if he engaged himself in passing through the stages of external and internal purifications, and used the formal and outer purifications as a means of purifying the spirit and the heart, and in all the acts of worship and rites he observed their spiritual aspects and was benefited by them, or better, if he gave more importance to the internal aspects and regarded them to be the most high objective, he would be admitted through the door of the suluk along the road of humanity, as is referred to in a noble hadith in Misbahush Shar'iah: "... And purify your heart with taqwa (fear of Allah) and yaqin (certitude) when you cleanse your organs with water." 
So, a salik man needs first a scientific suluk so as to distinguish, with the blessings of "the people of remembrance" [ahl-i dhikr] (SA), the stages of servitude, and regard the formal worship inferior to the spiritual and inner worship. Then the practical suluk, which is the reality of the suluk is started. The aim of this suluk is to free the soul from other than Allah, and adorn it with the manifestations of His Names and Essence. Getting to this stage, the salik would be at the end of his journey, attaining to the goal of his progress to perfection, and acquiring the secrets of austerity and worship, as well as the delicacies of suluk. Those are the manifestations of Majesty which are the secrets of purity, and the manifestations of Beauty, which are the objective of other worships. To give the details is out of the capacity of these pages.
. Uyunu Akhbarir Rida, vol. 2, p. 104, ch. 34, hadith 1.
. Biharul Anwar, vol. 67, p. 241, quoting Jami'ul Akhbar, p. 117 (with a slight difference).
. Ilalush-Sharai', vol. I, p. 280, ch. 191, hadith 1.
. Ibid, vol. 2, p. 378, ch. 109, hadith 1.
. Surah Ta Ha: 121.
. Misbahush Shari'ah, ch. 10, on "Purity".