الكافي- ط الاسلامية, volume 2, page 39

7. Certain individuals of our people have narrated from Ali ibn ‘Abbas from Ali ibn Maysir from Hammad ibn al-Nusaybi who has said the following: “A man asked the scholar (a.s.) ‘O scholar, tell me which deed is more virtuous before Allah?’ He said, ‘It is the deed without which Allah does not accept any deed.’ He asked, ‘What is such thing?’ He said, ‘It is belief in Allah (besides Whom no one deserves to be worshipped). It (belief) is the highest in degree among the deeds, the most valuable among them (in virtue) and the topmost among them in reward.’ The man then said, ‘Please, tell me then about belief. Is it deeds or it is words without deeds?’ “The Imam said, ‘All of belief is deeds and parts of such deeds are words. Allah has made it obligatory as is explained in His book. Its light is clear, its evidence well established. The (Holy) book testifies to it for one, and calls one thereto.’ I (the narrator) then asked, ‘Explain it to me so I may understand.’ He (the Imam) said, ‘Belief is of degrees, conditions, levels and stages. Of belief there is that which is perfect and complete in perfection. Of belief there is that which is defective and the defect is clear. Of belief there is that which is heavier on the perfection side.’ The man then asked, ‘Does belief become perfect, decrease and increase?’ He said, ‘Yes, it does.’ The man then asked, ‘How does it happen?’ He said, ‘It is because Allah, the Most Blessed, the Most High, has written belief for each part of the body of the children of Adam and has divided and distributed it among them. There is no part of the parts but that has a guard of belief that is different from such guard as is assigned to other parts. Of such part is one’s heart with which one understands and rationalizes to comprehend, and he is the commander of one’s body. It oversees the body and without its opinion no part moves to act. Of the parts of the body are one’s hands with which one works, one’s legs with which one walks, one’s sexual organs from which come carnal desires, one’s tongue with which one speaks of the book and testfies thereby, one’s eyes with which one sees and one’s ears with which one listens. ‘For each of these parts a different obligation is written. The obligation of the heart is different from the obligation of the tongue. What is made obligatory for the tongue is different from the obligation of the eyes and what is made obligatory for the eyes is different from what is made obligatory for the ears. What is made obligatory for the ears is different from what is made obligatory for the hands and what is made obligatory for the hands is different from what is made obligatory for the legs. The obligation of the legs is different from the obligations of the sexual organs. What is obligatory for the sexual organs is different from the obligation of the face. ‘What is made obligatory for the heart of belief is affirmation, recognition and formation of belief, consent and submission to the fact that no one deserves to be worshipped except Allah Who is One and has no partner. He is the One and only Who deserves to be worshipped. He has not taken any companion and children. Muhammad (a.s.) is His servant and messenger.’”

Translation: Muhammad Sarwar·Thaqalayn link·The Arabic above is the original.