Imam Jafar al-Sadiq(as) said, in a long tradition, that Allah has enjoined faith (iman) upon the limbs of the son of Adam, and has distributed it and divided it evenly between them, and every single limb has been assigned a different portion of faith particular to it. I will divide the long hadith into smaller parts to narrate the duties of each limb.
HEART
The portion of faith with which the heart has been charged is to affirm, to know, to conclude a contract and willingly to admit and submit that there is no being worthy of worship but Allahسُبْحَانَهُ وَ تَعَالَى, the One and Only, who has no partner, the One God, who has taken neither wife nor son, and that Muhammad is His servant and messenger(pbuh). And it is to attest to every prophet and divine scripture that Allah has sent down. This affirmation and knowledge is the duty of the heart, upon which Allah has made it incumbent, as He has said in His Book:
'Not he who is compelled while his heart is at rest on account of faith' -16:106 and
'Now surely by Allah's remembrance are the hearts set at rest' - 13:28 and
'Those who say with their mouths "we believe" , while their hearts do not believe' - 5:41 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
So this is the affirmation and knowledge of faith that Allah has made incumbent upon the heart and that is its duty, and it is the fountainhead of faith.
FACE
He has made incumbent upon the face to bow down in prostration to Him in the night and in the day at prayer times, about which He has 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 a joint duty on the face as well as the hands and the feet, for He has said in another verse:
'And that the parts of prostration are Allah's, so call upon not anyone with Allah....' - 72:18