The following quotations of the scholars of Shariah regarding the precedence of the knowledge and science of Tasawwuf (Purification of the Self).
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 [tasawwaf] will be corrupted; and whoever studied Sufism and didn't study Jurisprudence will become a heretic; and whoever combined both will reach the Truth."
['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 with leniency 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 Ghazzali (450 - 505 AH.)
"I knew verily that Sufis are the seekers in Allah's Way, and their conduct is the best conduct, and their way is the best way, and their manners are the most sanctified. They have cleaned their hearts from other than Allah and they have made them as pathways for rivers to run receiving knowledge of the Divine Presence."
[al-Munqidh, p. 131].
Fakhr ad-Din ar-Razi (544 - 606 AH)
"The way of Sufis for seeking Knowledge, is to disconnect themselves from this worldly life, and they keep themselves constantly busy with Dhikrullah, in all their actions and behaviors."
['Itiqadaat Furaq al-Muslimeen, p. 72, 73]
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. 20]
Ibn Taymiyya (661 - 728 AH)
"Tasawwuf has realities and states of experience which they talk about in their science. Some of it is that the Sufi is that one who purifies himself from anything which distracts him from the remembrance of Allah and who will be so filled up with knowledge of the heart and knowledge of the mind to the point that the value of gold and stones will be the same to him. And Tasawwuf is safeguarding the precious meanings and leaving behind the call to fame and vanity in order to reach the state of Truthfulness, because the best of humans after the prophets are the Siddiqeen, as Allah mentioned them in the verse:
"(And all who obey Allah and the Apostle) are in the company of those on whom is the grace of Allah: of the prophets, the sincere lovers of truth, the martyrs and the righteous; Ah! what a beautiful fellowship." (an-Nisa', 69,70)
"...some people criticised Sufiyya and Tasawwuf and they said they were innovators, out of the Sunnah, but the truth is they are striving in Allah's obedience [mujtahidin fi ta'at-illahi], as others of Allah's People strove in Allah's obedience. So from them you will find the Foremost in Nearness by virtue of his striving [as-saabiq ul-muqarrab bi hasab ijtihadihi]. And some of them are from the People of the Right hand [Ahl al-Yameen mentioned in Qur'an in Sura Waqi'ah], but slower in their progress.... And this is the origin of Tasawwuf. And after that origin, it has been spread and [tasha'abat wa tanawa'at] has its main line and its branches.
[Majmu'a Fatawa Ibn Taymiyya al-Kubra, Vol. 11, Book of Tasawwuf, p. 497].
"The miracles of saints are absolutely true and correct, by the acceptance of all Muslim scholars. And the Qur'an has pointed to it in different places, and the Hadith of the Prophet (s) has mentioned it, and whoever denies the miraculous power of saints are only people who are innovators and their followers." [al-Mukhtasar al-Fatawa, page 603]. Ibn Taymiyya says, "what is considered as a miracle for a saint is that sometimes the saint might hear something that others do not hear and they might see something that others do not see, while not in a sleeping state, but in a wakened state of vision. And he can know something that others cannot know, through revelation or inspiration."
[Majmu'a Fatawi Ibn Taymiyya, Vol. 11, p. 314].
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 prayer Allah accepts and by means of whom Allah supports human beings"
[Mu'eed an-Na'am p. 190, the chapter entitled Tasawwuf]
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-'Aliyya,p 57]
Ibn 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) I 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."
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].
Maulana 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 their sins and to be away from every disobedience of Allah 'Azza wa Jall. Their guides were encouraging them to move in the way of perfect Love to Allah 'Azza wa Jall.
"...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."
[Muslims in India, p. 140-146]
Maulana Abul 'Ala Maudoodi (1321 - 1399 AH.)
"Sufism is a reality whose signs are the love of Allah and the love of the Prophet (saw), where one absents oneself for their sake, and one is annihilated from anything other than them, and it is to know how to follow the footsteps of the Prophet (s). ..Tasawwuf searched for the sincerity in the heart and the purity in the intention and the trustworthiness in obedience in an individual's actions."
"The Divine Law and Sufism: "Sufism and Shariah: what is the similitude of the two? They are like the body and the soul. The body is the external knowledge, the Divine Law, and the spirit is the internal knowledge."
[Mabadi' al-Islam, p. 17]