The first act of disobedience of our father Adam (AS) and our Mother Hawa (Eve) was in the act of eating………..Eating from the forbidden tree.
The unique demonstration of love for Allah SWT by man and woman is the act of refraining from eating for a specified time (fasting, including fard and nafl) for which Allah SWT has promised immeasurable reward. (Imam M. Sabke)
Allah SWT says and I paraphrase: Prayer is for the benefit of the Abd, but fasting is especially for Allah and the reward for fasting has no fixed limit.
Fasting is a gift of love reciprocated, held in trust by Allah Subhanawataa’la, only to be given on the Day of Judgment.
Allah subhanawataala says in the Quran:(English translation by Yusuf Ali)
7.019: “O Adam! dwell thou and thy wife in the Garden, and enjoy (its good things) as ye wish: but approach not this tree, or ye run into harm and transgression.”
007.020 : Then began Satan to whisper suggestions to them, bringing openly before their minds all their shame that was hidden from them (before): he said: “Your Lord only forbade you this tree, lest ye should become angels or such beings as live for ever.”
007.021 : And he swore to them both, that he was their sincere adviser.
007.022 : So by deceit he brought about their fall: when they tasted of the tree, their shame became manifest to them, and they began to sew together the leaves of the garden over their bodies. And their Lord called unto them: “Did I not forbid you that tree, and tell you that Satan was an avowed enemy unto you?”
SUPPLICATION OF ADAM (AS):
007.023 : They said: “Our Lord! We have wronged our own souls: If thou forgive us not and bestow not upon us Thy Mercy, we shall certainly be lost.”
Prophet Muhammad peace and blessings be upon him would fast (nafl) every monday and thursday and on the middle three days of the month.