"Verily, he who has purified the heart is successful and he who has despoiled it has lost."

A lot of people have misunderstandings about tasawwuf. Many think that it is something beyond Qur'an and Sunnah. Errant Sufis as well as the superficial ulema, although on the opposite ends of the spectrum, are together in causing this mistaken notion. Consequently the first group has shunned the Qur'an and Hadith while the second group has shunned tasawwuf. Actually, although the term tasawwuf, like many other religious terms in use today, evolved later, the discipline is very much part of the Shariah. The department of the Shariah relating to external deeds like salat and zakat is called "fiqh" while the one dealing with the internal feelings and states of the heart is called "tasawwuf". Both are commanded in the Qur'an. Thus while commanding Salat and Zakat, the Qur'an also commands gratefulness and love of Allah and condemns the evil of pride and vanity. Similarly, in the books of hadith, along with the chapters on Ibadat, trade and commerce, marriage and divorce, are to be found the chapters on riya (showoff), takabbur, akhlaq, etc. These commands are as much a mandatory requirement as the ones dealing with external deeds.

Further Reading: What is Tasawwuf? (See below) Bay'at in Shari'ah
Remembering the Days of Allah - Hamza Yusuf
The Seventeen Benefits of Tribulation - Hamza Yusuf
To Know Yourself is to Know Allah - AbuBakr Karolia
What is Tasawwuf? - Khalid Baig