Hadith: We find within ourselves thoughts that one of us would find too grave to talk about." He said: "Have you experienced that?" They said: 'Yes'. He said: "That is the clear faith

عَنْ أَبِي هُرَيْرَةَ رضي الله عنه قال:
جَاءَ نَاسٌ مِنْ أَصْحَابِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، فَسَأَلُوهُ: إِنَّا نَجِدُ فِي أَنْفُسِنَا مَا يَتَعَاظَمُ أَحَدُنَا أَنْ يَتَكَلَّمَ بِهِ، قَالَ: «وَقَدْ وَجَدْتُمُوهُ؟» قَالُوا: نَعَمْ، قَالَ: «ذَاكَ صَرِيحُ الْإِيمَانِ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 132]

Abu Hurayrah (may Allah be pleased with him) reported:
Some of the Prophet's Companions came to him and said: "We find within ourselves thoughts that one of us would find too grave to talk about." He said: "Have you experienced that?" They said: 'Yes'. He said: "That is the clear faith."
Sahih/Authentic. - [Muslim]

Explanation

A group of the Prophet's Companions came and asked him about thoughts that came to their minds, and they found it too grave to utter them due to their ugliness and repulsiveness. Thereupon, the Prophet (may Allah's peace and blessings be upon him) said: This which you experienced is the clear faith and certitude that prompts you to stave off what Satan throws into your hearts and to refrain from uttering it because you consider it to be too serious. As such, Satan could not dominate your hearts, unlike others whose hearts have been overcome by Satan, and he faces no pushback and resistance from them.