Imam Ali (as), in his will to his son Muhammad b. Hanafiyyah, said: 'O my son, do not say that which you do not know, and, at the same time, do not say everything that you do know, for Allah has enjoined certain duties on each and everyone of your limbs, which He will use to testify against you on the Day of Judgement, and He will ask you concerning them, as to how you heeded their counsel, their admonition, their warnings, and how you refined them, and He has not left them to wander aimlessly.
Allah, Mighty and Exalted, has said: 'And do not follow that of which you have no knowledge;surely the hearing and the sight and the heart, all of these shall be questioned about that' - 17:36
and He has said:'When you received it with your tongues and spoke with your mouths that of which you have no knowledge, and you deemed it an easy matter, while with Allah it was grievous'- 24:15
Then He bade them to worship Him by obeying Him, and said: 'O you who believe! Bow down and prostrate yourselves and serve your Lord, and do good that you may succeed' - 22:77
This is therefore a comprehensive duty incumbent on all our limbs, for he has also said: ' and that the parts of prostration are Allah's, so call not upon anyone beside Allah' - 72:18 By the parts of prostration, He means the face, the hands, the knees and the two big toes.
And Allah, Mighty and Exalted, has said: 'And you did not use to veil yourselves lest your ears and your eyes and your skins should bear witness against you' - 41:22 and by skin here He means the private parts.Then he singled out each limb for a specific duty, which He has clearly determined.
He has enjoined on the ears that they must not listen to that which is forbidden and constitutes disobedience to Allah, and He has said regarding this :'And indeed he has revealed to you in the Book that when you hear Allah's communications disbelieved and mocked, do not sit with such people until they enter into some other discourse, surely then you would be like them' - 4:140
from which He, Mighty and Exalted, has made an exception, i,e., when hearing such discourse is a result of forgetfulness: 'And if the Shaytan causes you to forget,then do not sit after recollection with the unjust people' -6:68
and He has said: 'Therefore give good news to My servants, those who listen to the word and follow the best of it;those are they whom Allah has guided and those it is who are the men of understanding' - 39:17,18
And: Successful indeed are the believers, who are humble in their prayers, and who keep aloof from vain talk, and who are givers of the poor rate - 23:1-4
and: 'When they hear idle talk they turn aside form it' - 28:55
and : 'When they pass by vain talk, they pass by it nobly' - 25:72
This is what Allah has enjoined upon the ears and, this is their duty.
And on the eyes, He has enjoined that they should not look at what Allah has forbidden, and regarding this, He, Mighty and Exalted has said: 'And say to the believing men that they cast down their looks and guard their private parts' - 24:30 and has forbidden anyone to look at private parts of another.
He has enjoined upon the tongue to attest to and to express that in which the heart has believed, with regards to which He has said: 'you shall speak good words to people' - 2:83 and 'Say, '' we believe in that which has been revealed to us and revealed to you, and our God and your God is One, and to Him do we submit" ' - 29:46
And he has enjoined upon the heart,which is the seat of authority for all the limbs, to perceive and to understand, and by whose command and conviction one sets about one's duties, and regarding this, He has said: 'not he who is compelled while his heart is at rest on account of faith, but he who opens his breast to disbelief - on these is the wrath of Allah, and they shall have a grievous chastisement' - 16:106.
Allah, Mighty and Exalted, with regards to a group of people who verbally proclaimed their faith while their hearts did not have conviction, says: 'Those who say with their mouths "we believe" , while their hearts do not believe' - 5:41 and 'Now surely by Allah's remembrance are the hearts set at rest' - 13:28 and 'Whether you make manifest what is in your hearts or hide it, Allah will call you to account for it; then He will forgive whom He pleases and chastise whom He pleases' - 2:284.
And he has enjoined upon the hands that they must not be used for anything that Allah, Might and exalted, has prohibited, and that they must be used to obey Him, as He has said:
'O you who believe!When you rise up for prayer, wash your faces and your hands as far as the elbows, and wipe your heads and your feet to the ankles' - 5:6
and: 'So when you encounter in battle those who disbelieve, then smite their necks until you have overcome them,then make them prisoners,and afterwards either set them free as a favour, or let them ransom themselves until the war ends' - 47:4
He has prescribed that the feet be used to walk towards that which is in obedience of Him, and that they should not be used to walk about insubordinately, with regards to which He has said: 'And do not go about in the land exultingly, for indeed you cannot cut through the earth nor reach the mountain in height':all this - the evil of it - is hateful in the sight of your Lord- 17:37,38
and 'On that day We will set a seal upon their mouths, and their hands shall speak to Us, and their feet shall bear witness of what they had earned' - 36:65.
Allah informs us that they will testify for or against those who had control of them on the Day of Judgement. 'This is the sum of what Allah has enjoined on your limbs, so be careful of your duty to Allah, my son, and use them towards His obedience and His pleasure, and beware of Allah, exalted be His name, watching you as you disobey Him, for the you will surely be one of the losers.' al-Faqih,v.2, p.381,no.1627.