The Prophet (saas) reminded people that Allah heard even their most secret conversations
some people think that when they intend to do something wrong, or plan a wicked act with others of like-mind, or gossip, they can conceal it from others. however, allah knows everything an individual thinks, what enters his mind, what two people whisper to each other, and everything in heaven and earth. he witnesses everything a person does. every individual will have to offer account in the hereafter for everything he thought he had done or said in secret. that person may perhaps forget those words full of wickedness, but allah will place everything he has forgotten before him on the day of judgment.
the prophet (saas) also drew attention to that fact, and warned people they would have to give account of themselves. these are some of the verses on the subject:
say, "whether you conceal what is in your breasts or make it known, allah knows it. he knows what is in the heavens and what is on earth. allah has power over all things." (surah al 'imran: 29)
say: "allah is a sufficient witness between me and you. he is certainly aware of and sees his servants." (surat al-isra': 96)
if they turn their backs, then say: "i have informed all of you equally and i do not know if what you have been promised is near or far. he knows what is said openly and he knows what you hide." (surat al-anbiya': 109-110)
say: "do you presume to teach allah your religion when allah knows everything in the heavens and everything in the earth? allah has knowledge of all things." (surat al-hujurat: 16)