It refers to the purity of one’s actions, that is, the outward aspect (zāhir) of a person being cleansed from opposition to divine commands. Undoubtedly, this is due to the purification of their inner secrets from all forms of turbidity.
As for the view that they are called Sufi because they wear ṣūf (woolen garments), this is one opinion. However, this group did not distinguish themselves by wearing woolen garments out of humility alone. They did not wear only wool, but also wore garments made from fabrics such as cotton. Moreover, if a person wears woolen clothing to make people believe he is a Sufi, sālik (traveler on the path), or ascetic (zāhid), and to gain respect and attention as a Sufi-mutasawwif-mustaswif/sufiyya, then this is a blameworthy act of ostentation (riyāʾ).
After the second century, as dunyā (the world) became more alluring and people grew attached to it, those who turned to worship were distinguished by the name sufiyya. They became known for acts such as asceticism (zuhd), withdrawing from people to be alone, and devotion to worship, as well as the states of ecstasy (wajd) they experienced as a result. This capacity for perception is what distinguishes humans from other living beings. Perception is of two kinds: the perception of knowledge and maʿrifa (experiential knowledge) in terms of certainty (yaqīn), supposition (ẓann), and doubt (shakk); and the perception of states (aḥwāl) such as joy, sorrow, contraction (qabḍ), expansion (basṭ), contentment (riḍā), anger, gratitude (shukr), and patience (sabr). The intellect of a ṣāḥib (possessor) mutasawwif arises from the soul (rūḥ) and from the perceptions, volitions, and states within the body. These are what distinguish humans from other beings. (Bughyat, 17)
Regarding the afflictions of the heart (qalb), it is characteristic of the Sufi to avoid things that kill the heart, such as vain and meaningless matters and excessive heedlessness (ghafla). For it has been experienced that such things cause the death of the heart. The entire effort of the faqir (the Sufi) is nothing other than keeping his heart alive against anything that prevents him from remembering Allah. For the human heart is like the center hole of a millstone; if the heart is corrupted (if the hole is damaged, so too is the millstone). If the stone has two holes, it becomes impossible for it to turn (thus, one should not divide the heart or its attention, for Allah has not created two hearts in a person’s breast). (Anwār, I, 85)
Junayd said: "The Sufi is like the earth; everything bad is thrown upon it, yet only good comes forth from it."
Sehl ʿAbdullah (Tustarī) said: "The Sufi is one purified from turbidity, filled with thoughts, who has turned to Allah by renouncing things that would please himself. In his eyes, gold and mud are the same..." (Nabhānī, II, 340)
There is disagreement regarding the origin of the name Sufi. Some relate it to the Ahl al-Ṣuffa (the poor among the Emigrants who lived at the Prophet’s Mosque) who lived during the time of the Prophet. Others relate it to those who, by turning their thoughts and aims with their hearts to Allah and being present before Him in this state, are associated with the first row (ṣaff al-awwal) in its original sense. It is also said to be related to ṣūf (wool), because the early generations generally wore wool as it was softer, more conducive to love, humility, and asceticism, and because it was the garment of the prophets.
It is reported that the Prophet rode a donkey and wore wool. It is also narrated from him: "About seventy prophets passed through al-Rawḥāʾ (a settlement between Mecca and Medina); they worshipped facing the Kaʿba, barefoot and wearing woolen garments." Another narration states: "On the day Allah spoke to Mūsā (peace be upon him), he was wearing a woolen cloak, woolen trousers, and a garment with woolen sleeves and other parts made of wool."
It is also related that Jesus (ʿĪsā) wore woolen and hairy garments, ate from trees, and spent the night on the ground when darkness fell, as a Sufi-mutasawwif-mustaswif/sufiyya.
Hasan al-Baṣrī said: "I saw seventy Companions (ṣaḥāba) who fought at Badr; their clothing was of wool. I say that this last view is a linguistic matter."