Punishment for Slander in Islam
Question

What is the prescribed punishment for slander?
Praise be to Allah.What is slander?
What is meant by slander with regard to the prescribed punishment is an accusation of zina (adultery or fornication) or homosexuality . The one who accuses a person of zina or vilifies him in such terms has slandered him and accused him of something abhorrent. 
Slander is prohibited , and is indeed a major sin, if the person slandered is chaste and innocent of zina. 
Allah says (interpretation of the meaning): 
“Verily, those who accuse chaste women, who never even think of anything touching their chastity and are good believers — are cursed in this life and in the Hereafter, and for them will be a great torment.” [An-Nur 24:23] 
As-Sa`di (may Allah have mercy on him) said: 
“Allah has issued a stern warning against making accusations against chaste women, as He says: “Verily, those who accuse chaste women”, i.e., those who refrain from immoral actions, “who never even think of anything touching their chastity”, and such things would never cross their minds, “and are good believers — are cursed in this life and in the Hereafter”, and a curse applies only in the case of major sins. 
The curse is confirmed because it is ongoing in this world and in the Hereafter. “and for them will be a great torment”. This is in addition to the curse, and shows that they are even further removed from divine mercy, and that His severe punishment will be upon them.” 
And the Prophet (peace and blessings of Allah be upon him) said: “Avoid the seven sins that doom a person to Hell.” It was said: What are they, O Messenger of Allah? He said: “Associating others with Allah (shirk); witchcraft; killing a soul whom Allah has forbidden us to kill, except in cases dictated by Islamic law; consuming orphans’ wealth; consuming riba; fleeing from the battlefield; and slandering chaste, innocent women.” (Narrated by Al-Bukhari (2767) and Muslim (89) 
Al-Hafidh Ibn Hajar said: “What is meant by the sins that doom a person to Hell are major sins.” 
It says in Fatawa Al-Lajnah Ad-Da’imah (22/76): 
“It is not permissible for a Muslim to slander his brother, and it is a major sin; he must repent from that and seek forgiveness from the one whom he slandered; if the latter does not pardon him, he has the right to demand his rights according to Shari`ah. 
And Allah is the Source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions.” 
Shaykh `Abd al-`Aziz ibn `Abdullah ibn Baz, Shaykh `Abd Ar-Razzaq `Afifi, Shaykh `Abdullah ibn Ghadyan 
Punishment for slander in Islam
The scholars unanimously agreed that it is obligatory to carry out the prescribed punishment on one who slanders a chaste person, and the punishment is eighty lashes, because Allah says (interpretation of the meaning): 
“And those who accuse chaste women, and produce not four witnesses, flog them with eighty stripes, and reject their testimony forever. They indeed are the Fasiqun (liars, rebellious, disobedient to Allah).” [An-Nur 24:4]
The wisdom behind the prescribed punishment for slander 

It prevents accusations of immorality.
It protects people’s honour from being transgressed and protects their reputations from being tarnished.
It prevents enmity and grudges; wars may break out because of slurs against people’s honour.
It prevents such things becoming part of public opinion and protects people from having to hear them
It prevents the spread of rumours among the believers, because when there are a lot of accusations and such talk becomes common and is easily spoken of, the foolish become bold enough to commit such actions.

So it is wise to prescribe the prescribed punishment for slander so that people will refrain from accusing one another of immoral actions. (See: Al-Mughni (12/383); Al-Muhalla (11/268, 269); Al-Majmu` (22/94, 98); Hashiyat Ibn Qasim (7/330); Ash-Sharh Al-Mumti` (14/278)
For more details, please see the following answers: 21576 , 381937 , and 199049 . 
And Allah knows best.