Hadhrat Hasan Basri (May Allah shower His Mercy on him) says: The person who wears wool to debase himself for Allah Most High, Almighty Allah increased the radiance of sight and the heart for that person and the person who does so for pride, etc. will be thrown into hell along with the rebellious folk.
Hadhrat Hasan ‘Ali Kharraz (May Allah shower His Mercy on him) explains this in a very concise, eloquent and ardent way: Sufi is a person who gains comfort fr…om Almighty Allah and has handed over all his affairs to Almighty Allah, who doesn’t pay attention to anybody other than Allah, who when in embroiled in difficulties doesn’t pay any attention to the secret within it, who when he (or she) turns to Allah Most High doesn’t turn away ever again and calamities don’t have any effect on that person. A Sufi is the one who doesn’t see any nonexistence after existence, whose existence is present & whose attributes are their veil. The heart being free of all hostility is also Tasawwuf.
In the opinion of Hadhrat Ghouse Azam Shaykh ‘Abdul Wadir Jeelani (May Allah shower His Mercy on him) Tasawwuf is to have a relation with Allah Most High which is rooted in truthfulness (Sidq), one should treat people kindly, the Book of Allah Most High and obedience to the Sunnah of the Holy Prophet (Sallallahu alaihi wa sallam) are the 2 wings with which one can fly towards Almighty Allah. One should desist from the calamities and blameworthy actions of the Self (Nafs). The person who has been purified by Allah Most High is a Sufi. Tasawwuf is beyond poverty (Faqr), poverty is also included in Tasawwuf. One should practice poverty only for the sake of Almighty Allah and it should be reap reward. Hadhrat Shibli (May Allah shower His Mercy on him) defines Tasawwuf as: Tasawwuf is to keep the heart clear of any thoughts of anyone/anything other than Allah.
A Sufi is a person who is cut off from the creation and attached to the Lord Almighty.
Abu ‘Abdullah Muhammad bin Khafi Shirazi (May Allah shower His Mercy on him) says: There was a time that Satan was under control of the Sufis. Now the times are such that the Sufis are under control of Satan. A Sufi is the one who follows the Holy Prophet (Sallallahu alaihi wa sallam) in all details and generalities and whose inward aspect is immersed with Almighty Allah.
Hadhrat Najmuddin Kubra (May Allah shower His Mercy on him) says: The Sufis are those who traverse the path of Almighty Allah. Their lives are the best lives, their path is the straight-most path and their manners are the most cultivated and loved. If the intellect of the intellectuals, the wisdom of the wise and the knowledge of the scholars of the Shariah all combine to get something better than the life and etiquette of the Sufis, the friends of Allah Most High, it is not possible for them to. All their outward and inward actions are drawn from the niche of Prophethood and there is no radiance on the earth from which one can draw illumination better than the radiance of Prophethood.