Sahih Muslim | Book 38 | Hadith Number 6682

Narrated by Ibn Mas'ud
Ibn Mas'ud reported that there gathered near the House three persons amongst whom two were Quraishi and one was a Thaqafi or two were Thaqafis and one was a Quraishi. They lacked understanding but wore more flesh. One of them said: 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).