In the Glorious Qur'an, God emphasises the purification and purity of the human soul as follows:

وَ الشَّمْسِ وَ ضُحاها وَ الْقَمَرِ إِذا تَلاها وَ النَّهارِ إِذا جَلاَّها وَ اللَّيْلِ إِذا يَغْشاها وَ السَّماءِ وَ ما بَناها وَ الْأَرْضِ وَ ما طَحاها وَ نَفْسٍ وَ ما سَوَّاها فَأَلْهَمَها فُجُورَها وَ تَقْواها قَدْ أَفْلَحَ مَنْ زَكَّاها وَ قَدْ خابَ مَنْ دَسَّاها
“I swear by the sun and its brilliance, and the moon when it follows the sun, and the day when it makes manifest the sun (and her beauty) , and the night when it covers the sun, and the heaven and Him who made it ,and the earth and Him who extended it, and the soul and Him who made it perfect, then He inspired it to understand what is right and wrong for it. He will indeed be successful who purifies it and he will indeed fail whoever pollutes and corrupts it.”(91:1-10).

So, after swearing eleven times, after so much emphasis, God declares that the person who purifies his soul will be successful and whoever pollutes and corrupts his soul will fail. On the Day of Judgement there will be two groups of people: those who are prosperous and happy because they purified their soul and those who are in an unfortunate position because they were careless and negligent of their soul.

Purification of the soul is a prerequisite for closeness to God. Indeed, the whole point of morality and spirituality is to purify one's soul. It is only then that the soul starts shining, receiving and reflecting utmost radiation and light from God. If we want to meet God, Who is the Most Pure, then we need to achieve purity. It is impossible to be polluted and then try to go towards God. If we want to go somewhere where the people are smart, well-dressed and beautiful, then we too need to make ourselves clean and tidy, we should put on good clothes and thus make ourselves somehow compatible with them. Otherwise they will say that we will spoil their gathering and damage their reputation.

One of the main tasks of all the Prophets (a.s.) and a major aim behind all their endeavours in teaching the divine message was to help people to purify their souls. Referring to the mission of the

Prophet Muhammad (s.a.w.), the Glorious Qur'an says: 
هُوَ الَّذي بَعَثَ فِي الْأُمِّيِّينَ رَسُولاً مِنْهُمْ يَتْلُوا عَلَيْهِمْ آياتِهِ وَ يُزَكِّيهِمْ وَ يُعَلِّمُهُمُ الْكِتابَ وَ الْحِكْمَةَ وَ إِنْ كانُوا مِنْ قَبْلُ لَفي‏ ضَلالٍ مُبينٍ
“He is the one who has sent amongst illiterate people an apostle from among themselves who recites to them His verses and purifies them and teaches them the Book and the wisdom.” (62:2).
لَقَدْ مَنَّ اللَّهُ عَلَى الْمُؤْمِنينَ إِذْ بَعَثَ فيهِمْ رَسُولاً مِنْ أَنْفُسِهِمْ يَتْلُوا عَلَيْهِمْ آياتِهِ وَ يُزَكِّيهِمْ وَ يُعَلِّمُهُمُ الْكِتابَ وَ الْحِكْمَةَ وَ إِنْ كانُوا مِنْ قَبْلُ لَفي‏ ضَلالٍ مُبينٍ
“Certainly God conferred a great favour upon the believers when He raised among them a Messenger from among themselves, reciting to them His verses and purifying them, and teaching them the Book and the wisdom, although before that they were surely in manifest error.”  (3:164).
كَما أَرْسَلْنا فيكُمْ رَسُولاً مِنْكُمْ يَتْلُوا عَلَيْكُمْ آياتِنا وَ يُزَكِّيكُمْ وَ يُعَلِّمُكُمُ الْكِتابَ وَ الْحِكْمَةَ وَ يُعَلِّمُكُمْ ما لَمْ تَكُونُوا تَعْلَمُونَ
“As We sent to you an Apostle from among yourselves, who recites to you Our signs, and purifies you, and teaches you the Book and wisdom, and teaches you what you did not know.” (2:151)

Thus we see that one of the tasks of the Holy Prophet (s.a.w.), in addition to reciting the Qur'an and teaching the Qur'an and wisdom, was to help us to purify our souls. Indeed, the appointment of

Prophet Muhammad (s.a.w.) for such tasks was an answer to the prayer of Abraham (a.s.) and Ishmael (a.s.) after they raised the foundations of the House (ka'bah):
وَ إِذْ يَرْفَعُ إِبْراهيمُ الْقَواعِدَ مِنَ الْبَيْتِ وَ إِسْماعيلُ رَبَّنا تَقَبَّلْ مِنَّا إِنَّكَ أَنْتَ السَّميعُ الْعَليمُ رَبَّنا وَ اجْعَلْنا مُسْلِمَيْنِ لَكَ وَ مِنْ ذُرِّيَّتِنا أُمَّةً مُسْلِمَةً لَكَ وَ أَرِنا مَناسِكَنا وَ تُبْ عَلَيْنا إِنَّكَ أَنْتَ التَّوَّابُ الرَّحيمُ رَبَّنا وَ ابْعَثْ فيهِمْ رَسُولاً مِنْهُمْ يَتْلُوا عَلَيْهِمْ آياتِكَ وَ يُعَلِّمُهُمُ الْكِتابَ وَ الْحِكْمَةَ وَ يُزَكِّيهِمْ إِنَّكَ أَنْتَ الْعَزيزُ الْحَكيمُ
“Our Lord! Accept from us; surely You are the Hearing, the Knowing….Our Lord! And raise up in them a Messenger from among them who shall recite to them Your verses and teach them the Book and the wisdom, and purify them; surely You are the Mighty, the Wise.” (2:127-129)

Just imagine how wise Abraham was! How lovely his supplication was! In three places in the Qur'an, God says that He has sent the Holy Prophet (s.a.w.) to do the same thing that Abraham (a.s.) and Ishmael (a.s.) had wanted: to recite for the people the verses of the divine Book, to teach them the divine Book and wisdom and to purify their souls. Of course, it must be God Himself who inspired them to pray in this way. God is so merciful that He first invites us to call Him, then He inspires us what to ask and then he answers our call and prayer.

Thus purification of the people was an important task for the Holy Prophet (s.a.w.) and, indeed, all the Prophets (a.s.). These verses clearly show the great significance of the task of purification of the soul. It is noteworthy that in the prayer of Abraham and Ishmael the request of teaching the Book and wisdom is mentioned before the purification, but in all the three places that God describes the mission of the Prophet Muhammad (s.a.w.), purification precedes teaching the Book and wisdom. This indicates the priority and great importance of purification. This also suggests that a prerequisite for learning the Book and wisdom is to be pure.

There are a number of sources of impurity. A major or the major source of impurity is the attachment to the materialistic life and worldly affairs to the extent that the Prophet Muhammad (s.a.w.) is quoted as saying:

حُبُّ الدُّنْيَا رَأْسُ كُلِّ خَطِيئَةٍ أَلَا تَرَى كَيْفَ أَحَبَّ مَا أَبْغَضَهُ اللَّهُ وَ أَيُّ خَطَإٍ أَشَدُّ جُرْماً مِنْ هَذَا
“The attachment to this world is the source for every wrong. Beware how the one who is attached to this world has loved what God dislikes. What wrong can be a greater crime than this?”

The materialistic world (dunya) is the least important and valuable thing in the sight of God. To be attached to it and make it one's ultimate end in one's life is a grave mistake and impurity. Therefore, one of the major treatments of this problem and a crucial means of purification of the soul is to ask people to give alms. In some twenty verses of the Qur'an, giving alms (al-zakat) is mentioned right after establishing prayer (iqamat a-salat). For example, Allah (swt) says in the Qur'an:

وَ ما أُمِرُوا إِلاَّ لِيَعْبُدُوا اللَّهَ مُخْلِصينَ لَهُ الدِّينَ حُنَفاءَ وَ يُقيمُوا الصَّلاةَ وَ يُؤْتُوا الزَّكاةَ وَ ذلِكَ دينُ الْقَيِّمَةِ
“And they were not commanded except to worship God, dedicating their faith to Him as men of pure faith, and to maintain the prayer, and pay the zakat. That is the upright religion.” (98:5)

Zakat is derived from the same root as tazkiyah (purification) i.e. za-ka-wa which means growth and purity. It has been suggested (Lisan al-'Arab, Vol. 14, p. 358) that the reason for calling alms “zakat” lies in the fact that paying zakat purifies one's money and possessions. It is also true that paying alms causes growth (namā) and blessing (barakah) in one's money and sustenance. It seems more reasonable to suggest that the main reason for calling alms “zakat” is that it helps in purifying the soul by getting rid of the love for this world. This is why God says to the Prophet Muhammad (s.a.w.):

خُذْ مِنْ أَمْوالِهِمْ صَدَقَةً تُطَهِّرُهُمْ وَ تُزَكِّيهِمْ بِها وَ صَلِّ عَلَيْهِمْ إِنَّ صَلاتَكَ سَكَنٌ لَهُمْ وَ اللَّهُ سَميعٌ عَليمٌ
“Take charity from their possessions to cleanse them and purify them thereby, and bless them. Indeed your blessing is a comfort to them, and God is all-hearing, all-knowing.” (9:103)
In this verse, instead of the term zakat, sadaqah (charity) is used. However, the same point is there: Giving money for the sake of God helps in purification of the giver. Elsewhere the Qur'an says:
الَّذي يُؤْتي‏ مالَهُ يَتَزَكَّى وَ ما لِأَحَدٍ عِنْدَهُ مِنْ نِعْمَةٍ تُجْزى‏ إِلاَّ ابْتِغاءَ وَجْهِ رَبِّهِ الْأَعْلى‏ وَ لَسَوْفَ يَرْضى‏
“He who gives his wealth to purify himself and does not expect any reward from anyone, but seeks only the pleasure of his Lord, the Most Exalted, and, surely, soon he will be well-pleased.” (92:18-21)

Thus when someone spends some money for the sake of God on things such as giving to the needy people or building places for common good such as Mosques, seminaries, schools and hospitals both giver and receiver benefit. However, the main beneficiary is the giver who is giving some money which is the least valuable thing in the sight of God and instead achieves purity and pleasure of God.

إِنَّما تُنْذِرُ الَّذينَ يَخْشَوْنَ رَبَّهُمْ بِالْغَيْبِ وَ أَقامُوا الصَّلاةَ وَ مَنْ تَزَكَّى فَإِنَّما يَتَزَكَّى لِنَفْسِهِ وَ إِلَى اللَّهِ الْمَصيرُ
“…You can only warn those who fear their Lord in secret, and maintain the prayer. Whoever seeks purification for himself, seeks purification only for his own sake, and to God is the return.” (35:18)
 

Conclusion
It is a basic fact that we must have self-control. There can be no spirituality without self-discipline. We cannot develop ourselves by simply doing what we wish and satisfying and gratifying our soul. Of course, Islam tells us that self-control is just a beginning. What we need to do is to transform our soul from one which has an interest in lower desires into a soul which instead has a yearning for good things. By training and purifying our souls, our soul itself becomes a helper and an assistant to us. A major task of the Prophets and in particular the Prophet Muhammad (s.a.w.) was to help people to purify themselves. The reason for such a great emphasis on self-purification is the fact that God is the Most Pure and the Most Perfect and it is only by purification of the soul that we can achieve our ambition of getting close to Him. One major way of purification is to get rid of attachment to the materialistic life by giving out one's own money for the sake of God.