Imam Sadiq (peace be upon him): Abu Amr al-Zubayri narrated: I said to Imam Sadiq (peace be upon him), "Can faith be at the level of perfection, and can it also decrease and increase?" He said, "Yes! It is so." I said, "How?" The Imam (peace be upon him) said, "Allah the Exalted made faith obligatory upon the limbs of the children of Adam (peace be upon him), and distributed faith among the parts of the body, and assigned to each a specific share. He allotted a portion of faith to each of the limbs, and every organ has a particular duty which another organ is not charged with. One of these limbs is the heart, by which he understands and perceives, distinguishes bad from good, and grasps its good and evil. The heart is the commander and ruler of his limbs and organs; whatever the heart commands, the parts of his body obey and do not disobey its order. Among those organs are his eyes, by which he looks at things, and he has ears by which he hears... Allah has also made certain things obligatory upon the ear, and that is that he should first restrain his ears from what the Lord has forbidden, and keep away from prohibited things, and not lend his ear to speech that provokes Allah's wrath [and refrain from frivolities and listening to useless talk]. Allah said in the Qur'an: 'And Allah has revealed [this ruling] to you in the Qur'an, that when you hear people denying and mocking the verses of Allah, do not sit with them until they engage in some other conversation!' (Nisa/140). But Allah has made an exception for the case where one errs due to forgetfulness, and said: 'And if Satan makes you forget, then never sit, after remembrance, with this wrongdoing people!' (An'am/68). And He said: فَبَشِّرْ عِبادِ الَّذِینَ یَسْتَمِعُونَ الْقَوْلَ فَیَتَّبِعُونَ أَحْسَنَهُ أُولئِکَ الَّذِینَ هَداهُمُ اللهُ وَ أُولئِکَ هُمْ أُولُوا الْأَلْباب [So give glad tidings to My servants, those who listen to the word, then follow the best of it; those are they whom Allah has guided, and those it is who are the men of understanding]. This is what Allah has made obligatory upon the ear, that it must abstain from lending itself to what is not lawful for it to hear, and these are the works of the ear [and are considered the faith of the ear], and it is a part of faith."