Is it forbidden to think about sins as if the one thinking about them is committing them?

A person is not punished for the evil whispers (waswasa) that his soul suggests to him, as long as he does not act upon them or speak them, due to the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah has forgiven for my nation what their hearts whisper to them, as long as they do not act upon it or speak of it," or "what their souls speak to them." It is a condition for this forgiveness that the whispering does not settle in the soul or that one does not firmly resolve to commit the sin. If the whispering settles and its owner resolves to act upon it, then that is forbidden.