Do you think that Allah hears as we speak? The other one said: He does hear when we speak loudly and He does not hear when we speak in undertones, and still the other one said: If He listens when we speak loudly, He also listens when we speak in undertones. It was on this occasion that this verse was revealed:" You did not conceal yourselves lest your ears, your eyes and your skins would stand witness against you" (xli. 22).
Sahih Muslim Book 51, Hadith 6