Because wearing wool (sūf) is the custom of the prophets and the emblem of the walī (friend[s] of God) and the pure (aṣfiyāʾ). There are many transmitted reports and narrations on this subject. When the Sufis were ascribed to their garments, this became a name that also encompassed their knowledge, conduct, spiritual state (ḥāl), and praiseworthy character traits. Do you not see that Allah Most High referred to the elite (khawāṣṣ) of the companions of Jesus (peace be upon him) in relation to the garments they wore as the Disciples (Ḥawāriyyūn)... They used to wear white garments. Allah Most High ascribed them to these garments they wore, rather than to the knowledge, deeds, or spiritual state (ḥāl) they possessed. In my view, the case of the Sufis is the same. Allah knows best. The Sufis were ascribed to their outward garments (and thus received this name), and not to the knowledge and spiritual state (ḥāl) they were adorned with. For wearing wool is the custom of the prophets and the ṣiddīqūn (the utterly truthful), and it is the emblem of those who devote themselves to worship. (Lumaʿ, 40)
Those who resolve their anxieties through their intellects, distance themselves from them with their hearts (qalb), and seek refuge in their Lord from the evil of their nafs (the lower self)... these are the Sufiyya (the Sufis).
When Dhu’l-Nūn al-Miṣrī was asked, "Who is the Sufi?" he replied: "He is the one who does not tire of seeking, nor is he angered when something is forcibly taken from his hand." On another occasion, he said: "They are those who have preferred Allah over everything, and Allah has preferred them over everything." (Lumaʿ, 45)
When Abū Ḥusayn Aḥmad Muḥammad Nūrī was asked, "Who is the Sufi?" he said: "The Sufi is the one who listens to samāʿ (spiritual audition) and prefers to cling to causes (asbāb)."
When Abū Ḥasan Kannād was asked about the meaning of Sufi, he replied: "The word Sufi is derived from ṣafāʾ (purity). It is to fulfill the condition of loyalty to Allah at every moment." (Lumaʿ, 46)
When the meaning of Sufi was asked to another Sufi, he said: "The Sufi is the one who, by realizing the attribute of ʿubūdiyya (servitude), attains the friendship of the Real (al-Ḥaqq), is purified from the turbidity of human nature, thus reaching the degrees of ḥaqīqa (inner reality) and drawing near to the rulings of the sharia (the sacred law) (i.e., to the realities of these rulings). If one does this, then he is a Sufi. For he has been purified (by the Real who purifies the background)."
(I said:) In reality, who are the Sufiyya? Describe their qualities to us! He replied to me: "They are those who know Allah and His rulings, act according to what Allah Most High has taught them, realize what Allah has demanded from them, become enraptured by what they have realized, and are annihilated (fanāʾ) in what they experience in this ecstasy. For every person of ecstasy (wajd) is annihilated in that which he experiences ecstasy."
When Shiblī was asked, "Why are the Sufiyya called by this name?" he replied: "They are called so because of the remnants of their nafs (the lower self) that remain upon them. If it were not so, no name would be found or given that would be suitable for them."
sufi-mutaṣawwif-mustaṣwif/Sufiyya 923 ...It has also been said that the Sufiyya are the remainder of the People of the Suffa. (Lumaʿ, 47)
One of the shaykhs, when asked what Sufism (taṣawwuf) is, gave three answers: The first is according to the condition of knowledge, which is the purification (tasfiya) of the heart (qalb) from turbidity, good conduct toward created things, and following the Prophet (peace be upon him) in matters of the sharia (the sacred law). The second is an answer in the language of reality (ḥaqīqa), which is the absence of ownership, being freed from servitude to attributes, and sufficing with the Creator of the heavens. The third, as an answer in the language of the Real (al-Ḥaqq), is this: He purifies them by recognizing them by their attributes, cleanses them, and thus they are called Sufi. (Lumaʿ, 48)
(Some of the Sufis have said:) The Sufiyya are called so because their secrets (inner selves) are pure and their outward forms are clean.
Bishr Ḥāris said: "The Sufi is the one who has purified his heart (qalb) for Allah."
One of the Sufis said: "The Sufi is the one who, because he has purified his dealings with Allah, has attained the pure generosity of the Most High Allah."
Some Sufis say: "The Sufiyya are so called because they are in the first row (ṣaff) in the presence of the Most High Allah." Indeed, the Sufis have turned to the Real (al-Ḥaqq) with their spiritual aspiration (himma), directed themselves to Him with their hearts (qalb), and are present before Him with their spirits (ruh).
According to other Sufis, the Sufis are called Sufiyya because their qualities are close to those of the People of the Suffa in the time of the Messenger of God (peace be upon him). According to another group of Sufis, they are called Sufi because they wore garments made of wool (ṣūf, meaning wool). Those who ascribe the Sufis to the words suffa (platform) and ṣaff (row) have given them this name by considering their outward states.
The Sufiyya are a group who, abandoning the world (dunyā), leave their homelands; forsaking their friends, they wander from land to land; they leave their stomachs hungry and their bodies naked; and they do not benefit from the blessings of the world (dunyā) beyond what is absolutely necessary to cover their private parts and to relieve their hunger.
Because the Sufis have left their homelands, they are called "strangers" (ghurabāʾ); because they wander from land to land, they are called "travelers" (sayyāḥīn); and because they wander in the wilderness and, in cases of necessity, take refuge in caves, some of the people of certain lands have called them "shikeftiyya". In the language of those who call the Sufis shikeftiyya (the people of Khurasan), shifkeft means "cave". The people of Syria have called the Sufis "juʿiyya" (the hungry ones). The fact that the Sufis benefit from foodstuffs only to the extent of necessity has caused them to be given this name.
Describing the Sufiyya, Sari Saqatī said: "They eat like sick people, sleep like those who are drowning, and speak like those who are ashamed." Because they have abandoned the possessions in their ownership, the Sufis are also called "the poor" (fuqarāʾ). (Again Sari) 924 sufi-mutaṣawwif-mustaṣwif/Sufiyya
When he was asked, "Who is the Sufi?" he replied: "The Sufi is one who is neither an owner (mālik) nor owned (mamlūk). That is, greed cannot approach him." Another said: "The Sufi is one who owns nothing. If he does own something, he immediately spends it."
Because of their clothing and attire, the people of Sufism (taṣawwuf) are called Sufiyya. They do not wear soft and beautiful-looking garments for the pleasure of the nafs (the lower self). Because they wear garments only to cover their private parts, they are content with garments made of coarse hair and thick wool. (Taʿarruf, 6)
Bundār Ḥusayn said: "The Sufi is the one whom the Real (al-Ḥaqq) has chosen for Himself, purified and cleansed from his nafs (the lower self), and has not returned to the state of acting with affectation as a claimant. It is also possible to explain Sufi as being on the pattern of ufʿila (he was made pure): thus, Sufiyya means 'those who have been made pure.' Expressions such as: 'Āfāhu’Allāh fa-ufiya (Allah granted him well-being, so he found well-being); Kafāhu’Allāh fa-kufiya (Allah sufficed him, so he became sufficient); Jazāhu’Allāh fa-juzīya (Allah punished him, so he was punished)' are examples of this. In this ḥāl (spiritual state), Allah’s act regarding the Sufi is manifest in the name Sufi. The Sufi is the one who is alone with Allah."
When Abū ʿAlī Rūzbārī was asked, "Who is the Sufi?" he said: "Purity (ṣafāʾ)."