Do you get thoughts so evil that you dare not utter them? That is Imaan
And We have already created man and We know what his soul whispers to him, and We are closer to him than [his] jugular vein.
Qur'aan 50:16
It is narrated on the authority of Abu Huraira that some people from amongst the Companions of the Prophet (sallallaahu alayi wa sallam) came to him and said:
Verily we perceive in our minds that which every one of us considers it too grave to express. He (the Holy Prophet) said: Do you really perceive it? They said: Yes. Upon this he remarked: That is the faith manifest.
Sahiih Muslim Hadiith No. 132
Narrated Abu Huraira:
The Prophet (sallallaahu alayhi wa sallam) said, "Allah has forgiven my followers the evil thoughts that occur to their minds, as long as such thoughts are not put into action or uttered."
And Qatada said, "If someone divorces his wife just in his mind, such an un-uttered divorce has no effect.
Sahiih al-Bukhaari Hadiith No. 5269
Narrated Abdullah ibn Abbas:
A man came to the Prophet (sallallaahu alayi wa sallam) and said: O Messenger of Allah! one of us has thoughts of such nature that he would rather be reduced to charcoal than speak about them. He said: Allah is Most Great, Allah is Most Great, Allah is Most Great. Praise be to Allah Who has reduced the kaid (deception, deceit, plot, stratagem) of the devil to evil prompting.
Ibn Qudamah said "reduced his 'amr (affair, matter)" instead of "reduced his kaid (deception, deceit, plot, stratagem)".
Sunan Abi Daawud Hadiith No. 5112
Allaah says in the Qur'aan in Suurah al-Nisaa portion of Ayah No. 76
"... Indeed, the kaid (deception, deceit, plot, stratagem) of Satan has ever been weak."
Narrated `Ali bin Husain:
Safiya bint (daughter of) Huyai came to the Prophet (in the mosque), and when she returned (home), the Prophet (sallallaahu alayi wa sallam) accompanied her. It happened that two men from the Ansar passed by them and the Prophet called them saying, "She is Safiya!" those two men said, "Subhan Allah!" The Prophet (sallallaahu alayhi wa sallam) said, "Satan circulates in the human body as blood does."
Sahiih al-Bukhaari Hadiith No. 7171
Ibn Taymiyyah (paraphrase) said:
قال شيخ الإسلام ابن تيميَّة - رحمه الله - في "الفتاوى": "والمؤمِنُ يبتلى بوساوس الشيطان وبوساوس الكفر، التي يضيق بها صدره، كما قالتِ الصحابة: يا رسول الله، إن أحدنا ليجد في نفسه ما لأن يخر من السماء إلى الأرض أحب إليه من أن يتكلم به، فقال: ((ذاك صريح الإيمان))، وفي رواية: ما يتعاظم أن يتكلم به، قال: ((الحمد الله الذي رد كيده إلى الوسوسة))؛ أي: حصول هذا الوسواس مع هذه الكراهة العظيمة له، ودفعه عن القلوب - هو من صريح الإيمان؛ كالمجاهد الذي جاءَهُ العدو، فدافعه حتى غلبه، فهذا عظيم الجهاد... ولهذا يوجدُ عند طلاب العلم والعباد من الوساوس والشبهات ما ليس عند غيرهم؛ لأنه لم يسلك شرع الله ومنهاجه، بل هو مقبل على هواه في غفلة عن ذكر ربه، وهذا مطلوب الشيطان، بخلاف المتوجهين إلى ربهم بالعلم والعبادة، فإنه عدوهم يطلب صدهم عن الله تعالى". اهـ
The believer is tried by means of the evil thoughts from the devil and by the evil thoughts of kufr, thoughts which cause his chest to constrict (i.e. he feels horrible) just like the Companions said:
It is narrated on the authority of Abu Huraira that some people from amongst the Companions of the Prophet (sallallaahu alayi wa sallam) came to him and said:
Verily we perceive in our minds that which every one of us considers it too grave to express. He (the Holy Prophet) said: Do you really perceive it? They said: Yes. Upon this he remarked: That is the faith manifest.
Sahiih Muslim Hadiith No. 132
To get these thoughts and then have this horrible distaste for them and try to expel them from the heart is what is Sariihul Imaan (manifest faith). It is like the Mujaahid to whom the enemy comes and he repels them until he conquers his enemy. This is aziimul jihaad (magnificant Jihaad).
That is why you find that the seekers of knowledge and worshipers suffer from waswaas (evil whispers) and doubts which others do not experience. This is because others have not walked the path Allaah has legislated for them. Rather, they are running after their desires while being in heedlessness from the remembrance of their Lord. And this is what the devil wants as opposed to the ones who are constantly engaged in knowledge and worship seeking Allaah. Indeed, he is an enemy to them who wants to distract them from Allaah.
Reference(s):
Tafsir al-Tabari
Tafsir Ibn Kathiir