Sahih Muslim Book 1, Hadith Number 239.

Chapter : Pertaining to evil suggestion or prompting in faith and what should be said when it occurs to the mind of a man.

It is narrated on the authority of Abu Huraira that some people from amongst the Companions of the Apostle (may peace be upon him) 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.