Allah clearly states that Sihr is forbidden in His Book, where He (Exalted be He) says: "And they followed what the Shayâtîn (devils) gave out (falsely of the magic) in the lifetime of Sulaimân (Solomon). Sulaimân did not disbelieve, but the Shayâtîn (devils) disbelieved, teaching men magic and such things that came down at Babylon to the two angels, Hârût and Mârût, but neither of these two (angels) taught anyone (such things) till they had said, "We are only for trial, so disbelieve not (by learning this magic from us)." And from these (angels) people learn that by which they cause separation between man and his wife, but they could not thus harm anyone except by Allâh's Leave. And they learn that which harms them and profits them not. And indeed they knew that the buyers of it (magic) would have no share in the Hereafter. And how bad indeed was that for which they sold their ownselves, if they but knew. And if they had believed and guarded themselves from evil and kept their duty to Allâh, far better would have been the reward from their Lord, if they but knew!"
Allah (Glorified be He) points out that Sihr is Kufr and that it is from the teachings of the devils. Allah has condemned them for that, and they are our enemies. He states that teaching Sihr is Kufr and that it is harmful and not beneficial, so we must beware of it.
Learning Sihr is all Kufr, hence He states that the two angels did not teach it to people until they had told them, "We are for trial, so disbelieve not (by learning this magic from us)." Then He says: but they could not thus harm anyone except by Allâh's Leave. So it is known that it is Kufr and misguidance and that the practitioners of Sihr cannot harm anyone except by Allah's Leave.
What is meant is His universal Qadari will (i.e. things that He decrees should happen even though He may dislike them), not His religious Shar`y will (i.e. that which He prescribes and which pleases Him) – because Allah has not prescribed this and has not given permission for it
Then Allah (Glorified be He) says: And how bad indeed was that for which they sold their ownselves, if they but knew. This means that they have sold themselves to the devils for this Sihr. Then, Allah (Glorified be He) says: And if they had believed, and guarded themselves from evil and kept their duty to Allâh, far better would have been the reward from their Lord, if they but knew! This indicates that learning Sihr and using it is the opposite of faith and piety. There is neither might nor power except with Allah!