What type of religious innovation (bid'ah) is the origin of Sufism?

Question:
He says: Honorable Shaykh, may Allah grant you success. You mentioned to us that the origin of Sufism is, in its essence, a religious innovation (bid'ah), but it is a light religious innovation. Are there any religious innovations with this description?

Answer:
Yes, it contains a light religious innovation. It is not a single thing. I told you that it is either shirk (associating partners with Allah), or kufr (disbelief), or fisq (open sinfulness), or a sin. For example, the one who fasts on the middle day of Sha'ban; this is a religious innovation. Do you say that he is an open sinner? You say this is a sin, because it is not established from the Prophet (peace and blessings be upon him) that he prescribed fasting on the middle of Sha'ban. So do not say that he is an open sinner. Do not say that it is a sin; it is contrary to the Sunnah. Religious innovations differ. Yes.