Know that the niyah [intention] is to decide or to determine to do something. It is the soul's decision on performing some act after conceiving it and then acknowledging its advantage and judging its necessity. It is a psychological and conscientious condition which appears after the said procedures. It is, then, expressed as to be a decision, a determination, a want, a will, an objective and the like. It appears in all voluntary actions, as there can be no voluntary act without undergoing the said process, and it is there in the entire action, in reality, not allegorically. It does not need, however, that the details should be in the mind from the very beginning or even during the process, nor should one necessarily imagine the objective and the decision in detail. It sometimes happens that man does the act according to a decision, and yet he is completely unaware of the detailed picture of the act and the decision, while the fact is there, and it takes place in the outside motivated by that fact. This is quite consciously obvious in the voluntary acts.
In sum, this decision and determination, which is the niyah in the terminology of the faqihs [ may Allah be pleased with them], is, inevitably in every act, such that if one wanted to do a voluntary act without it, it would not be possible. Nevertheless, the whisperings of the wicked Satan and the sportings of fancy overrule reason and disguise the necessities in the eyes of the helpless man, and man, instead of spending his precious life in improving and purifying his deeds, and in freeing them from internal evils, and instead of spending it in acquiring monotheistic knowledge, godliness and being in quest of Allah, the vile Satan whispers in his ears and induces him to spend only half of his life in quest of a necessary and obligatory matter. Satan's snares and artifices are too many: one may be induced by him to give up the act altogether, while the other whom he despairs of inducing to drop the act, induces him to commit other follies, such as self-conceitedness and hypocrisy. If Satan could not succeed in this, he would try to falsify one's worship by way of causing him to pretend holiness: by slighting the worships of the people in his eyes, and by causing him to regard the people indifferent [heedless]. Then he induces him to spend all his life on matters such as the niyah , which is inseparable from the act , or the takbir or the recitation , which are common and ordinary acts . At any rate , Satan would not leave man alone before nullifying his worship by one way or another.
Satanic whisperings come through diverse ways, which we cannot discuss for the time being , nor scrutinize them all. But the whispering in the niyah, from among them, may be the most ridiculous and the strangest, because if someone tried, with all his powers, to do a voluntary act in all his life, without a niyah [an intention], it would be impossible for him to do it. Nevertheless, you may find some wretched, sick and feeble-minded person who spends a considerable time at every salat just to have a firm intention. Such a person is more like the one who thinks a long time whether he is to decide to go shopping or to go for a lunch. This helpless man, to whom the salat should be his ascension to His proximity, and the key to his happiness, and, by applying its cordial disciplines and realizing the secrets of this divine grace he should perfect his essence [dhat] and secure his growth [nash'ah] of life, he, on the contrary, neglects all these matters, or, much worse, besides regarding them unnecessary, he takes them all to be false, and uses his dear and valuable capital to serve Satan and to obey his slinking whispers, placing the God-given reason [`aql], which is the light of guidance, under the rule of Iblis.
`Abdullah ibn Sinan says: "I mentioned before Imam as-Sadiq (AS) [a case concerning] a man who was afflicted with frequently performing the wudu' and the salat, and described him to be a man of reason. The Imam said: "What reason has he in obeying Satan?' I asked him: `How is it that he is obeying Satan?' He said: `If you ask him wherefrom his hesitation comes to him, he will tell you it is of the work of Satan."' 
In short, man should uproot this by whatever means of austerity and strife, for it deprives him of all happiness and good. It may be that forty years of a man's worship be not correctly performed even in the outer appearance or according to the juristic formal details, let alone the internal religious disciplines. More ridiculous is that some such fastidious persons think that the worships of all people are invalid and take them to be negligent in respect of religion, whereas their religious authorities-whose instructions they follow-are also among the ordinary people. If he is a learned man, let him refer to the Prophet's tradition to see that the Messenger of Allah (SA) and the Imams of guidance (AS) were also like the ordinary people in these matters. From among all the people only this group of fastidious persons, act contrary to the Messenger of Allah (SA), the infallible Imams (AS), the jurists of religion and the scholars of the `ummah, regarding the acts of worship of all of them worthless, believing their own to be done according to precaution and that they take good care of religion. Take, for example, the case of the wudu'. The narratives about the wudu' of the Messenger of Allah (SA) are many and successively transmitted. It seems that the Messenger of Allah (SA) used a single handful [of water] to wash his face, another for his right hand and a third for his left hand.  The Imami faqihs are unanimous on definitely regarding this wudu' to be the correct one. The Book of Allah , too, apparently confirms it. As to the second washing, or the second handful, it is objected by some, while there is no harm in the second handful, or the second washing, although its recommendation is questionable. But the third washing is undoubtedly an innovation and renders the wudu' invalid, according to both the narration and the fatwa. Now, look at the acts of this helpless fastidious person who is not satisfied with twenty handfuls [of water], each one of which is enough to fully wash the hand and to consider it a " Complete Wash". In this case his wudu' is undoubtedly invalid. But this wretched, feeble-minded person, who performs this act in obedience to Satan's orders and whispers, thinks that what he does is correct and according to precaution, then, by contrast, he takes the acts of all others to be incorrect and invalid. Here the noble hadith proves true in respect of calling this man insane. The one who thinks correct an act which is contrary to the act of the Messenger of Allah (SA), and regards an act which is in conformity with the Prophet's to be incorrect, is either a renegade or an insane. But as this desperate person is not a renegade, he must be insane, obedient to Satan and disobedient to the Beneficent [rahman].
There is no remedy for this acute disease except through contemplation and reflection on the afore-mentioned matters, making a comparison between his act and that of the pious, the scholars and the faqihs [may Allah be pleased with them]. If he sees that his act differs from theirs, he is to push Satan's nose into the dust and to completely ignore that evil creature. Then, after that if Satan tries and whispers in his ears several times that: "Your act is invalid," let him reply: "If the acts of all the faqihs of the 'ummah are invalid, mine is invalid, too." It is hoped that after some time spent contradicting Satan, taking refuge with Allah, the Exalted, with invocations and imploring, from his evils, this disease will be cured and Satan's greedy eye will be lifted from him. The same instructions are also recommended in the noble hadiths to remedy excessive doubt, which is also of the Satanic inspirations.
In the noble al-Kafi, on the authority of Imam al-Baqir (AS),it is stated that he has said: "When doubts in the salat become too frequent, do not give heed to them , and regard the salat to be correct . It is hoped that this state would leave you, as it is not caused but by Satan." 
In another narrative, Imam al-Baqir (AS) , or Imam as-Sadiq (AS) , is quoted to have said: "Do not make it Satan's habit to frequent you by breaking your salat, as this will stir his greed against you. Satan is evil and gets accustomed to what he is habituated to."
Zurarah says that the Imam added: "That malignant wants to be obeyed. So, if he is disobeyed he will not come back to you." 
This is an important cure for all cases of Satan's intrusions and of the jestings of devilish fancies.In some hadiths invocations are also recommended. You may, in this respect, refer to Wasa'ilush-Shi`ah, Mustadrakul Was'il at the end of book of al-khalal.
. 'Usulul Kafi, vol. 1, p. 13, "Book of Reason and Ignorance", hadith 10.
. Furu `ul Kafi, vol. 3, p. 24, "Book of Purity", ch. on the "Description of the Wudu".
. Ibid., p. 359, "Book of the Salat", ch. on "The One Who Doubts in his Salat", hadith 8.
. Ibid., hadith 2.