Of the important cordial disciplines of worships, especially the invoking worships, one is tranquillity [tuma'ninah], which is not the same tranquillity as demanded by the jurisprudents (may Allah be pleased with them) in the salat. It is that the salik should perform his worship with a quietude of the heart and a tranquillity of the mind, because if the worship was performed in a state of anxiety and with shaky heart, the heart would not have any reaction, and no effects of worship would appear in the dominion of the heart, and the reality of the worship would not become an inner image of the heart.
One of the significances of the repetition of worships and the increase of supplications and invocations is that the heart is effected by them and there will be an emotion until, gradually, the reality of worship and supplication forms the innermost part of the salik, and his heart unites with the spirit of worship. Should the heart lack tranquillity [itminan], quietude and serenity, there would be no effect of the supplications and the rituals on it, and they would not pass from the outer appearance and limits of the body to the dominion of the inner soul, and the cordial parts of the truth of worship would not be effected. This is one of the clear matters that need no explanation, as it could be understood by a little contemplation.
If a worship was such that the heart could have no information whatsoever about it, and there was no effect at all in the interior, it would not be kept in other dominions, nor would it ascend from the kingdom of this world to the kingdom of the heavens. And it may be that at the times of difficulties of the illness of death and its horrible agonies, and the terrors and calamities of after death, the image of such a worship may completely be effaced -God forbid! -from the plane of the heart, and that man may stand at the sacred presence of Allah empty-handed. For example, if one recites the noble supplicatiton: La ilaha illallah, Muhammad Rasulullah (There is no god but Allah; Muhammad is the Messenger of Allah) with a calm and tranquil heart and teaches his heart to repeat it, the tongue of the heart will gradually become talking, and the outer tongue will be a follower of the heart's tongue, and then the heart will supplicate first, and the tongue will follow it. This is referred to by Imam as Sadiq (AS) as is stated in Misbahush- shari'ah (the Torch of the Islamic Law). He said: "Let your heart be your tongue's qiblah ,and do not move it except by the heart's order, the reason's assent and the faith's consent," 
At the beginning, before the tongue learns talking, the salik on the way to the Hereafter, is to teach it and to instruct it, in tranquillity and quietude, with the supplications. As soon as the heart finds its tongue, it becomes the qiblah (the focus of attraction) of the mouth's tongue and of the other organs of the body. When it starts supplication, the whole kingdom of the human existence becomes supplicant. But if the noble supplication is recited without the tranquillity and calmness of the heart, but with haste, trouble, and unbalanced senses, it will have no effect on the heart, and will not pass the limits of the animal tongue and ear of the outer body to the interior and the human hearing, and its truth will not be implemented in the core of the heart, nor will it become a perfect and non-transitory image of the heart. Thus, when there are terrors and difficulties, especially the terrors of the agonies of death and the difficulties of the throes of the last breaths, one completely forgets supplication and it will be erased from the plane of the heart. Even the name of Allah the Exalted, the Seal of the Prophets (SA), the noble religion of Islam, the divine sacred Book, the Imams of guidance (AS) and other knowledge [ma'arif] which have not been conveyed to the heart, will be forgotten, and when he is questioned in the grave, he cannot answer, and the talqin will be of no avail, because he does not find in himself any trace of the truth of Lordship, prophethood or other knowledge [ma'arif], and that what he used to chatter about, and had no image in his heart, vanishes from his memory, and thus he will have no share of testifying the Lordship, prophethood and other knowledge [ma'arif].
It is stated in a hadith that a group of the people [ummah] of the Messenger of Allah (SA), on being entered into Hell, forget the name of the Prophet because of the fearfulness of the Hell-keeper, despite the fact that the same hadith states that they are of the believers and their hearts and features glitter brightly because of the light of faith. 
The great narrator, the late Majlisi, in Mir'atul 'uqul explaining the expression: "I will be his hearing and his seeing", says: "One who does not dedicate his eyes, ears and other organs to the way of obeying Allah, will not obtain spiritual eyes and ears, as his corporeal eyes and ears will not go to the other world, and so he will be without ears and eyes in the worlds of the grave and the Resurrection, whereas the criterion for the questionings of the grave is those spiritual organs." (the end of the sum of his translation). 
In short, the noble hadiths about this kind of tranquillity and its effects are many. The Glorious Qur'an orders that it (the Qur'an) should be recited in tartil, and it is stated in the noble hadiths that: "Whoever forgets a surah from the Qur'an, it will appear before him in Paradise in a most beautiful image. When he looks at it he says: 'What are you? How wonderful you are! I wish you were mine'. It will reply: 'Do you not know me? I am so-and-so surah. If you had not forgotten me, I would have lifted you to this high rank."' 
In another hadith it is stated that: "Whoever learns the Qur'an as a young man, it will mix with his flesh and blood." This is because the heart of a young man is less engaged and less tarnished, and so, it is affected quicker and easier, and the effect remains longer.
There are many hadiths in this respect, to which we shall refer when we discuss "recitation", insha 'allah. In a noble hadith it is said: "Nothing is more loved by Allah than a deed which is done persistently, no matter how small the deed."  Its important point may be that such a deed becomes the inner image of the heart, as it has already been noted.
. Misbahush-Shari'ah, ch. 5 "On Supplication", and Mustadrakul Wasa'il, "Book of as-Salat", chs. on "Supplication", ch. on "Rarities", hadith 2.
. 'Ilmul Yaqin, vol. 2, p. 1039.
. Mir'atul 'Uqul, vol. 10, p. 392.
. 'Usul ul Kafi, vol. 4, p. 410, "Book of the Merit of the Qur'an" ch. "One Who learnt the Qur'an then forgot it," hadith 2.
. Ibid., p. 405, ch. "The Merit of the Carrier of the Qur'an", hadith 4.
. Ibid., vol. 3, p. 137, "Book of Faith and Infidelity", ch. "Economy in Worshipping", hadith 2.