From Sahih Muslim (Hadith 7029):
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)
Reference: Muslim 7029