Allah Most High says,
Those who slander chaste secured, and believing women are cursed in this life and in the hereafter: for them is a grievous chastisement. On the day when their tongues, their hands, and their feet will bear witness against them as to their as to their actions. (Nur: 23-24)
He also says,
And those who launch a charge against chaste women and produce not four witnesses (to support their allegations) flog them with eighty stripes: and reject their evidence ever after: for such men are wicked transgressors. (Nur: 4)
Allah the Almighty in these verses makes it clear that whosoever unjustly accuses as chaste free woman of committing adultery is cursed both in this world and in the hereafter, and deserves grave chastisement.
In addition, he is flogged with eighty lashes and his witness is not accepted even though he is just.
In the Sahih Bukhari and Muslim the Prophet says,
“Avoid seven heinous sins.....”
And among which he mentioned charging believing women, unmindful though innocent, with adultery"
An example of such an accusation is if someone says to a Muslim chaste free woman:" You are an adulterer." or "a harlot", or "a whore" or to say to her husband, “you are the prostitute's husband" or to call her child as " the whore's child".
The same rule is applied if someone slanders a man of committing adultery, or a free boy with pederasty. The unjust accuser in all the above cases must suffer eighty stripes unless he brings a testimony of four men who support his allegation. Otherwise, he must be flogged with eighty lashes upon the request of the offended person.
The Prophet, too, forbade a man to wrongfully accuse his bondman or his slave girl of committing adultery.
A person who accuses his maid servant with adultery, will be subjected to the punishment of slandering on the Day of Judgement unless it happens to be true."
A lot of ignorant people are indulged in such offenses which procure the chastisement in this world and in the hereafter.
When a person says something unknowingly right or wrong, it may lead him to fall down in hell as far away as the distance between the east and west.
Muadh bin Jabal asked,
"O Messenger of Allah! Will we be called to account for what we say?" The Prophet answered “May you be lost to your mother! The people will be thrown with their face down, into the hell only on account of the product of the tongues" i.e. their talking: "He who believes in Allah and the Day of Resurrection should speak well. Otherwise, he should keep silent.” [Bukhari and Muslim]
Not a word does he utter but there is a vigilant Guardian. (50:18)
Uqbah Ibn Amir once asked the Prophet (pbuh),
"What is the salvation, Allah's Apostle?" The Prophet said, "Control your tongue, keep inside your house, and weep (be sorry) for your sins. And the person farthest from Allah will be the one whose heart is hard.” [Tirmidhi]
"Verily Allah dislikes the indecent obscene person." [Tirmidhi]
Ascribing Associates To Allah, the Most High (Shirk)
Killing A Human Being
Not Performing The Prayer
Not Paying Zakat
Breaking One's Fast During Ramadan Without An Excuse
Not Performing the Hajj when Able to
Showing Disrespect to One's Parents
Severing the Ties of One's Relatives
Accepting Usurious Gain
Wrong Consuming An Orphan's Property
Lying About The Prophet
Fleeing From The Battlefield
The Leader Who Misleads His Followers, The Tyrant And The Oppressor
Arrogance, Pride, Conceit, Vanity And Haughtiness
Bearing False Witness
Accusing A Woman Of Adultery
Misappropriating Spoils Of War, Muslim Funds Or Zakat
Highwaymen Who Menace The Road
The Engulfing Oath
Taking People's Property Through Falsehood
The Consumption of Haram
The Dishonest Judge
Women Imitating Men And Vice Versa
The Pimp and the one Who Permits His Wife to Fornicate
Marrying Solely To Return To The Previous Husband
Not Freeing Oneself Of All Traces Of Urine
Showing Off In Good Work
Learning Sacred Knowledge For The Sake Of This World, Or Concealing It
Breach of Trust
Reminding Recipients of Ones Charity To Them
Disbelieving In Destiny (qadar)
Listening To People's Private Conversations
The Talebearer Who Stirs Up Enmity Between People
Breaking Promises or Pledges
Believing Fortune Tellers And Astrologers
Wife Rebelling Against Her Husband