The word Sufism is an english word with arabic roots, in Arabic the science is called Tasawwuf, and the act you perform is called Tazkiyah an Nafs, Purifying your self of evil and bad while promoting good qualities, and what you try to achieve by it is Ihsan, Human Perfection.
Tasawwuf can be established with Three verses of the Qur’an and a Hadith:
1) In Surah al Shams (91:1-9) Allah takes an oath in the Qur’an like no other and swears by the act of Tasawwuf, Allah says “Qad Aflaha man Zakaha” the word Zakaha means Tazkiya an nafs, in english the sentence reads “He Who Purifies it (his self) succeeds”. This verse is talking about getting rid of your bad qualities, Tazkiya an Nafs, and the one who does it will be granted Jannah (heaven).
2) In Surah Abbasa (80:3) a blind man comes to the Prophet (saws) seeking to learn about Islam and the Prophet (saws) frowns upon His untimely question because He is busy in a meeting with the leaders of Quraish, Allah responds to the prophet (saws) by saying, “Wama yudrika La-alahu Yazaka” the word Yazaka also means Tazkiyah an Nafs, in english the verse reads, “Yet for all thou didst know,
So in Surah a Shams Allah mentions the first part of purifying the self (Tazkiyah an Nafs) and that is Purifying your self of Evil, and in surah Abbasa Allah mentions the second part of tazkiya, which is learning and promoting good qualities in your character and growing as a person. The word zakaha means “purifies it” referring to our selfs, which is the first step, and the word yazaka means to “grow in purity”or to build upon the first step of purifying the evil from our self.
3) In Surah Ashurah (26:88-89) Allah says, “The Day whereon neither wealth nor sons will avail, But only he (will prosper) that brings to Allah a sound heart”, that is “a heart free of sins and love for this worldly life” (Tanwir al miqbas Min Tafsir Ibn Abbas).
You have to know the acts of Tazkiyah an Nafs (purifying your character) to free your Heart from sin and love for this world, and only one science in Islam deals with this Subject, that is Tasawwuf, Fiqh does not cover Human Character and Psychology, it covers the Laws of Halal (the permitted) and Haram (the prohibited), it may touch upon it because it uses the same Qur’an and sunnah but that isn’t it’s focus or aim. How to actually change your self, the Ullumah dealt with that in detail in a different science of Islam.
4) The most famous hadith in Islam, the hadith of Jibril (as), which occurred 86 days before the prophets (saws) death it defines all the Aspects of islam with out a Doubt, into Fiqh (Islam), Aqeedah (Iman), Ihsan (Tasawwuf) and the Sings of the Hour, we have already mentioned the hadith and it is a sound, Sahih Hadith mentioned by Imam Muslim and Imam Bukhari as well as many other Muhadiths in their works.
Ihsan, Perfection is achieved when a person gains the ability to worship Allah as if he is seeing Him every hour in our life, and this is done through Tazkiyah an nafs (Purification of the Self). Allah says about human Perfection in the Qur’an “We sent unto her Our Spirit (Jibril) and it assumed for her the likeness of a perfect man.” (19:17), the Angels can take on the form of any man that is sound and healthy but here Allah is stressing that Jibril (Gabriel) chose to show Maryam (as) the likeness of a perfect Human being whose presence alone is not like other men, this was so she could recognise him as a person from Allah.
All the Scholars of Islam in our history knew about tasawwuf and most of the great Scholars practised it because they wanted to achieve human perfection and it was through this practice that Allah granted them knowledge we are benefiting from today. About their practice of tasawwuf Allah says in the Qur’an “So for this let the competitors compete.” (83:26), encouraging the Ummah to out do each other in gaining nearness to Him (Aza wa Jaal) by perfecting the self. The verse itself is referring to illiyyun which is the Highest part of the highest level of Jannah (heaven) just beneath the Throne of Allah.
The List of Ullumah who practiced Tazkiyah an Nafs is extensive and the argument over Tasawwuf by those properly educated in it, is one over which practice in tasawwuf is acceptable in Islam and which isn’t, this is Usually Ikhtilaf (difference of legal opinion) between the Hanbali Madhab, which is literalist in its understanding, and the other three Madhhabs of Islam. Not wether tasawwuf is part of Islam or not, which is kufr to claim it isn’t since it is mentioned in the Qur’an similar to how Fiqh and Aqeedah are mentioned.
The Subject of Ihsan dates back to the time of the messenger himself (saws) who spoke about it extensively and this is reflected in the major Hadith collections.
The following works are part of the Sahih Sita, Islam’s major hadith collections and are chapters found in each scholars work, they cover an important part of tasawwuf and the sunnah:
Imam Tirmidhi’s Book of Asceticism, Imam Muslims Book of Zuhd (Asceticism) Piety And Softening Of Hearts, Imam Muslims’ The Book of Manners and Etiquette (Adab), Imam Bukhari’s Book of Good Manners and Form (Al-Adab), Imam Bukhari’s Book “To Make The Heart Tender”, Imam Malik’s Book on Good Character, Imam Ibn Majah’s Book of Zuhd (Asceticism), Imam Abu Dawwud’s Book of General Behaviour (Kitab Al-Adab), Imam Bukhari’s Al-Adab Al-Mufrad (on Conduct and behaviour).
All these books mention Ahadith about the prophets (saws) Life that are Important to Tasawwuf.
This understanding that Tasawwuf is part of Islam is established clearly in the history of the Hanbali madhhab, Ibn Taymiyah himself was a Qadiri Sufi and died a Qadiri Sufi, his Major work majmool al Fatawa, which is 36 volumes, the 11th Volume is entitled al Tasawwuf in which he teaches tasawwuf, he has an essay in it called al-Sufiyya wa al-fuqara. In his commentary Ibn Taymiyya stresses that the primacy of the Shari`a (its essence at the start of islam) forms the soundest tradition in tasawwuf, and to argue this point he lists over a dozen early masters, as well as more contemporary shaykhs like his fellow Hanbalis. In his al-Risala al-safadiyya, Ibn Taymiyya defends the Sufis as those who belong to the path of the Sunna and represent it in their teachings and writings.
It is Kufr (disbelief) to deny a single verse in the Qur’an, so denying that the verses on Ihsan or Tazkiyah an Nafs exist itself isn’t anything less than Kufr.
Allah in Surah al Shams, 91 verses 1 – 10 takes an Oath on thirteen things He created, like no other oath in the Qur’an, except when He swore on the life of our prophet (saws), that Tazkiya an Nafs, which the science of Tasawwuf deals with, is the aim of every Human and Jinn in this life.
Allah said;
By the Sun and his (glorious) splendour;
By the Moon as she follows him;
By the Day as it shows up (the Sun’s) glory;
By the Night as it conceals it;
By the Firmament and its (wonderful) structure;
By the Earth and its (wide) expanse:
By the Soul, and the proportion and order given to it;
And its enlightenment as to its wrong and its right;-
Truly he succeeds that purifies it,
And he fails that corrupts it! (The Soul or Nafs).
It is clear from the Qur’an itself that the Ullumah did not neglect the topic of Ihsan over the past 1400 years because it is the thing all creatures being tested on this earth by Allah aim for, “He Who created Death and Life, that He may try which of you is best in deed” (67:2).