Imam al-Qushayri wrote al-Risala, a foundational text presenting tasawwuf as discipline of the heart within Sunni theology and law. It maps stages of the soul and corrects excess while affirming the Sunna.
The work is a standard reference for understanding why classical fiqh and tasawwuf were not opposed but intertwined.
Is Sufism allowed in Islam?
SeekersGuidance
Traditional definition of tasawwuf in Sunni Islam.