According to the early Islamic psychologists, the ruh is a non-material reality which pervades the entire human body, but is centred on the heart, the qalb. It represents that part of man which is not of this world, and which connects him with his Creator, and which, if he is fortunate, enables him to see God in the next world. When we are born, this ruh is intact and pure. As we are initiated into the distractions of the world, however, it is covered over with the ‘rust’ (ran) of which the Quran speaks. This rust is made up of two things: sin and distraction. When, through the process of self-discipline, these are banished, so that the worshipper is preserved from sin and is focussing entirely on the immediate presence and reality of God, the rust is dissolved, and the ruh once again is free. The heart is sound; and salvation, and closeness to God, are achieved.
This sounds simple enough. However, the early Muslims taught that such precious things come only at an appropriate price. Cleaning up the Augean stables of the heart is a most excruciating challenge. Outward conformity to the rules of religion is simple enough; but it is only the first step. Much more demanding is the policy known as mujahada: the daily combat against the lower self, the nafs. As the Quran says:
‘As for him that fears the standing before his Lord, and forbids his nafs its desires, for him, Heaven shall be his place of resort.’ 79:40
Hence the Sufi commandment:
‘Slaughter your ego with the knives of mujahada.’ al-Qushayri, al-Risala (Cairo, n.d.), I, 393.
Once the nafs is controlled, then the heart is clear, and the virtues proceed from it easily and naturally.
Because its objective is nothing less than salvation, this vital Islamic science has been consistently expounded by the great scholars of classical Islam.
Source: Islamic Spirituality: The Forgotten Revolution (https://ruhaniqalb.wordpress.com/2016/06/27/islamic-spirituality-the-forgotten-revolution/)