After Hadhrat Nabi Aadam (Alayhis salaam) had mistakenly ate from the forbidden tree in Jannat, he sincerely repented. However, his Taubah was not accepted for 30 days because the athr (effect) of the haraam fruit was still in his bodily system. The athr was eliminated only after 30 days. When he was totally purified even physically of the evil effects of the forbidden fruit, his Taubah was accepted. Thus, 30 days fasting became the practice and speciality of the Ummat of Rasulullah (Sallallahu alayhi wasallam).
This is the view proffered by the Sufiya (Auliya). Haafiz Ibn Hajar (Rahmatullah alayh) claims that there is no authentic Sanad for this narration of the Sufiya. But his view is not the final word. The illustrious Sufiya who were great Auliya made no false claims. They were not fabricators of Ahaadith. Ibn Hajr’s view is consequently simply set aside. While the views of the Sufiya revealed by way of Kashf and Ilhaam are not Shar’i Dalaa-il on which ahkaam could be formulated, they may not be dismissed as fabrications and baseless.