Does the shaytaan know what crosses our minds of ideas which no one knows except Allah and thus whisper to us what is in accordance with our thoughts to instigate us in harboring negative thoughts and committing sins?.
Iblis whispers to mankind to lead them astray.
From the evil of the retreating whisperer - Who whispers [evil] into the breasts of mankind -
This whispering is speech that Iblis intends to be hidden from Allah, but of course, that is not possible.
But do they not know that Allah knows what they conceal and what they declare?
Iblis uses ones' own thoughts against them, instilling doubts and fears.
One of the harmful effects of waswasa of thought is that the affected person always takes the negative view of things. Some people don’t see anything positive about themselves or those around them. They take a negative and morose view of everything. They never think of the strengths they or those around them have, but are always complaining about their failings and weaknesses. ... Another symptom of waswasa of thought is that the person gets fed up with life and remains sad. If the person is pious, he wonders why he has come into this world. He thinks it would have been better if he had not come into the world. ... The worst effect of waswasa of thought is that the person becomes suspicious. First he is plagued with bad thoughts about his own family, and then gradually this extends to the society. If this malady progresses further, he starts doubting Allah, the Holy Quran, the Prophet (s) and his Progeny (a.s.).
However, the whispers of Iblis are just about meaningless.
Those who believe fight in the cause of Allah , and those who disbelieve fight in the cause of Taghut. So fight against the allies of Satan. Indeed, the plot of Satan has ever been weak.
And if an evil suggestion comes to you from Satan, then seek refuge in Allah. Indeed, He is Hearing and Knowing. Indeed, those who fear Allah - when an impulse touches them from Satan, they remember [Him] and at once they have insight.
So, looking closely at the ways that Iblis tries to lead mankind astray, we see rebelliousness, weakened faith, increased pride, conceit, laziness, Muslim-to-Muslim fighting, and disregarding obligatory deeds. Unfortunately, no human is immune from these thoughts at times, no matter how pious they are. Iblis has been watching humanity since its creation. He has had enough time to learn how humans think.
Iblis only knows what is in every human's mind, but he uses that to trick us into thinking that he sees into our hearts. Only Allah truly knows our thoughts.
Praise be to Allaah.
Evil whispers during prayer and at other times come from the Shaytaan, who is keen to misguide the Muslim and deprive him of good and keep it far away from him. One of the Sahaabah complained to the Messenger of Allaah (peace and blessings of Allaah be upon him) about waswaas during prayer, and he said: “The Shaytaan comes between me and my prayers and my recitation, confusing me therein.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “That is a devil called Khanzab. If he affects you seek refuge in Allah from him and spit drily to your left three times.” He [the Sahaabi] said, I did that and Allaah took him away from me.
I had same issue after I read this I got cure alhamdulillah
Answers to this question cannot be verified. This question existed many centuries ago, many scholars had different (and contradicting) views.
I went by an arabic fatwa stating some of those views. You may use a translator if you wish.