Here are a few links about Sufism
http://www.sufiway.net/sec4=ORigSUF811326.html
http://www.nuradeen.com/Reflections/ElementsOfSufism1.htm
http://sunnah.org/tasawwuf/tasawwuf.htm
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A few very important scholars of Islam on Sufism
Imam Abu Hanifa (85 H. - 150 H) "If it were not for two years, I would have perished." He said, "for two years I accompanied Sayyidina Ja'far as-Sadiq and I acquired the spiritual knowledge that made me a gnostic in the Way." [Ad-Durr al-Mukhtar, vol 1. p. 43]
Imam Malik (95 H. - 179 H.) "whoever studies Jurisprudence (tafaqaha) and didn't study Sufism [tasawwafa] will be corrupted; and whoever studied Sufism and didn't study Jurisprudence will become a heretic; and whoever combined both will be reach the Truth." [the scholar'Ali al-Adawi , vol. 2, p 195.)
Imam Shafi'i (150 - 205 AH.) "I accompanied the Sufi people and I received from them three knowledges: ... how to speak; .. how to treat people withleniency and a soft heart... and they... guided me in the ways of Sufism." [Kashf al-Khafa, 'Ajluni, vol. 1, p 341.]
Imam Ahmad bin Hanbal (164 - 241 AH.) "O my son, you have to sit with the People of Sufism, because they are like a fountain of knowledge and they keep the Remembrance of Allah in their hearts. they are the ascetics and they have the most spiritual power." [Tanwir al-Qulub p. 405]
Imam Nawawi (620 - 676 AH.) "The specifications of the Way of the Sufis are ... to keep the Presence of Allah in your heart in public and in private; to follow the Sunnah of the Prophet (s) ... to be happy with what Allah gave you..."[in his Letters, (Maqasid at-tawhid), p. 201]
Ibn Khaldun (733 - 808 AH.) "The way of the Sufis is the way of the Salaf, the preceding Scholars between the Sahaba and Tabi'een of those who followed good guidance..." [Muqaddimat ibn al-Khaldun, p. 328]
Tajuddin as-Subki (727 - 771 AH.) "May Allah praise them [the Sufis] and greet them and may Allah cause us to be with them in Paradise. Too many things havebeen said about them and too many ignorant people have said things which are not related to them. And the truth is that those people left the world and were busy with worship. ... They are the People of Allah, whose supplications and player Allah accepts and by means of whom Allah supports human beings" [Mu'eed an-Na'am p. 190, the chapter entitled Tasawwufl
Jalaluddin as-Suyuti (849 - 911 AH.) "At-Tasawwuf in itself is the best and most honorable knowledge. It explains how to follow the Sunnah of the Prophet (s) and to put aside innovation." [Ta'yid al-Haqiqat al-'Aiiyya,p 57]
lbn Qayyim (691 - 751 AH.) "We can witness the greatness of the People of Sufism, in the eyes of the earliest generations of Muslims by what has been mentioned by Sufyan ath-Thawri (d. 161 AH), one of the greatest imams of the second century and one of the foremost legal scholars. He said, "If it had not been for Abu Hisham as-Sufi (d. 115) 1 would never have perceived the action of the subtlest forms of hypocrisy in the self... Among the best of people is the Sufi learned in jurisprudence." [Manazil as-Sa'ireen.]
Abdullah ibn Muhammad ibn Abdul Wahhab (1115 - 1201 AH.) "My father Muhammad ibn Abdul Wahhab and I do not deny or criticize the science of Sufism, but on the contrary we support it, because it purifies the external and the internal of the hidden sins, which are related to the heart and to the outward form. Even though the individual might externally be on the right way, internally he might be on the wrong way. Sufism is necessary to correct it." [ad-Dia'at mukathaffa did ash-Shaykh Ibn Abdul Wahhab,p.85 ]
Ibn 'Abidin (1198 - 1252 AH.) "the Seekers in this Sufi Way don't hear except from the Divine Presence and they don't love any but Him. If they remember Him they cry, and if they thank Him they are happy; ... May Allah bless them." [Risa'il Ibn'Abidin p. 172 & 173]
Muhammad 'Abduh (1265 - 1323 AH.) "Tasawwuf appeared in the first century of Islam and it received a tremendous honor. It purified the self and straightened the conduct and gave knowledge to people from the Wisdom and Secrets of the Divine Presence." (Majallat al-Muslim, 6th ed. 1378 H, p. 24].
Abul Hasan 'Ali an-Nadawi (1331 AH b.) "These Sufis were initiating people on Oneness and sincerity in following the Sunnah of the Prophet (s) and to repent from theirsins and to be away from every disobedience of Allah 'Azza wa Jail. Their guides were encouraging them to move in the way of perfect Love to Allah 'Azza wa Jail. "...In Calcutta India, everyday more than 1000 people were taking initiation into Sufism. "...by the influence of these Sufi people, thousands and thousands and hundreds of thousands in India found their Lord and reached a state of Perfection through the Islamic religion."[Muslit-ns in India, p. 140-146]
Ibn Taymiah on Sufism
Ghazali on Sufism
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Sufi cosmology and psychology
Muraqaba