Sometimes I cant control my thoughts like something really horrible or haram come in my mind accidentally and suddenly. Is it the wasswassa of shaytan? Is it considered as a gunnah?

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Yes of course this is waswasa of Satan try to cut and discontinue it whenever these type of thoughts come in your mind but there is no sin just for your accidental thoughts untill you commit a sin practically, related to that bad thought. Recite 'La hawla wala quwwata illa billahil Aliyyil Azeem" whenever such thoughts come in your mind.