كتاب الإيمان
1
The Book of Faith
(60)
Chapter: Clarifying the Waswasah (Whispers, Bad Thoughts) with regard to faith, and what the one who experiences that should say
(60)
باب بَيَانِ الْوَسْوَسَةِ فِي الإِيمَانِ وَمَا يَقُولُهُ مَنْ وَجَدَهَا ‏‏

It is narrated on the authority of Abu Huraira that some people from amongst the Companions of the Apostle (ﷺ) 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.
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنْ سُهَيْلٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ جَاءَ نَاسٌ مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم فَسَأَلُوهُ إِنَّا نَجِدُ فِي أَنْفُسِنَا مَا يَتَعَاظَمُ أَحَدُنَا أَنْ يَتَكَلَّمَ بِهِ ‏.‏ قَالَ ‏"‏ وَقَدْ وَجَدْتُمُوهُ ‏"‏ ‏.‏ قَالُوا نَعَمْ ‏.‏ قَالَ ‏"‏ ذَاكَ صَرِيحُ الإِيمَانِ ‏"‏ ‏.‏
Reference : Sahih Muslim 132 a
In-book reference : Book 1, Hadith 247
USC-MSA web (English) reference : Book 1, Hadith 239
  (deprecated numbering scheme)