Through the initial physical challenge of the fast, the soul is agitated and its level of maturity is tested. In this way, the physical fast is a means to an inner, spiritual fast. The fast ultimately reveals to you everything that comes between you and Allah s.w.t., every tendency to break down and lapse out of trust in Allah s.w.t. when placed under pressure. How you respond to this discomfort determines the degree of success of your spiritual fast.
—Ilyas al Kashani, on the purification of the fast (sawm) during Ramadan
(h/t Maryam Eskandari)