Islam has taken a firm and decisive stance against Zina (fornication or adultery). Allah, the Almighty, commands in explicit and unequivocal words: (And come not near unto adultery. Lo! it is an abomination and an evil way) (Al-Isra’ 17: 32).
thus, Islam not only prohibits Zina, but also closes all the avenues and means leading to it. This is achieved by prohibiting every step and means leading to stimulating desires, opening ways for illicit sexual relations between men and women, and promoting indecency and obscenity.
In his response to the question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:

Adultery in Islam is one of the most heinous and deadliest of sins. Its enormity can be gauged from the fact that it has often been conjoined in the Qur’an with the gravest of all sins: shirk or associating partners with Allah.
the enormity of this sin is no small measure due to its dire consequences affecting individuals, families, and societies. Among these are that it entails infidelity and erodes the trust and tranquility that are the foundations of a fulfilling family life; it dissipates one’s energies; it undermines peace at home; it corrodes the purity of one’s soul and hence destroys one’s faith; finally, it exposes the person to the wrath of Allah, thus resulting in eternal damnation.
It is no wonder then that Allah and His Messenger have sounded dire warnings against adultery in so many ways. To list only a few instances:
Allah says in the Qur’an, (And come not near unto adultery. Lo! it is an abomination and an evil way) (Al-Isra’ 17: 32).
(And the servants of the Beneficent) are those who do not invoke another god with Allah, and who do not…commit fornication/adultery, for whoever does that shall receive the penalty; for him shall the torment be doubled on the day of resurrection, and therein he shall abide forever, disgraced, save him who repents and believes and does good works; those, Allah shall change their misdeeds into good works. And Allah is Forgiving, Compassionate) (Al-Furqan: 68-70).
the Prophet (peace and blessings be upon him) said, “Whoever guarantees me that he will guard his chastity, I will guarantee him Paradise” (Al-Bukhari).
abu Hurayrah reports that the Messenger of Allah said, “No one commits adultery while still remaining a believer, for faith is more precious unto Allah than such an evil act!” In another version, it is stated, “When a person commits adultery he casts away from his neck the bond that ties him to Islam; if, however, he repents, Allah will accept his repentance” (Al-Bukhari, Muslim, Abu Dawud, An-Nisa’i and others).
the Prophet (peace and blessings be upon him) further said: “[Under Islamic laws in an Islamic state] It is not lawful to shed the blood of a Muslim except for one of three sins: a married person committing fornication, and in just retribution for premeditated murder, and [for sin of treason involving] a person renouncing Islam, and thus leaving the community [to join the enemy camp in order to wage war against the faithful].” (Al-Bukhari, Muslim, Abu Dawud, Tirmidhi, and An-Nasa’i)
During his ascension, the Prophet (peace and blessings be upon him) was escorted by Jibreel and shown different types of punishments meted out to those guilty of various major offenses. While recounting the various scenes shown to him, he said, “Then Jibreel took me and we passed by a group of people with terribly inflated bodies emitting worst foul smells just like that of open sewers; when I enquired who they were, he replied, ‘These are those who commit adultery!’” (IbnKhuzaymah)
the Prophet (peace and blessings be upon him) said: “Faith is like a shirt that Allah places on those He chooses; but whoever commits adultery his shirt will be taken off from him; if he were to repent sincerely, Allah will place it back on him.” (Al-Bayhaqi)
“There are three types of sinners that Allah will not speak to on the Day of Resurrection; neither He will purify them nor will He even look at them; rather they will suffer severe punishment: an older person who commits adultery, a king or ruler who lies to his subjects, and a poor person who acts arrogantly!” (Muslim and An-Nasa’i)
“When adultery becomes rampant in a nation Allah will expose them to His chastisement.” (Abu Ya`la)
“When promiscuous behavior becomes rampant in a nation, Allah will send upon them such (strange) diseases that their own ancestors never heard of.” (At-Tabarani)
“There are seven categories of people who will be granted protection under the shade of Allah on the Day when there is no other shade;… among them is a young person who is seduced by a woman of great beauty and wealth and yet he rejected her advances saying ‘I fear Al
lah!’” (Al-Bukhari and Muslim)
the Prophet (peace and blessings be upon him) said, “O mankind! Beware of fornication/adultery for it entails six dire consequences: three of them relating to this world and three to the next world. As for the three that are related to this world, they are the following: it removes the glow of one’s face, brings poverty, and reduces the life-span. As for its dire consequences in the next world they are: it brings down the wrath of Allah upon the person, subjects him to terrible reckoning, and finally casts him in hell-fire” (Al-Bayhaqi).

Given the above, it is to be noted that the punishment specified for an unmarried person guilty of fornication in the Shari`ah is 100. In the very beginning of Surat An-Nur, it is stated that: (The woman and the man guilty of adultery or fornication,- flog each of them with a hundred stripes: Let not compassion move you in their case, in a
matter prescribed by Allah, if ye believe in Allah and the Last Day: and let a party of the Believers witness their punishment) (An-Nur 24: 2).

As for the punishment specified in the Shari`ah for the married adulterer or adulteress, it is stoning to death. In the Kuwaiti Encyclopedia of Islamic Jurisprudence, we read the following:

IbnQudamah wrote: “Muslim jurists are unanimous on the fact stoning to death is a specified punishment for married adulterer and adulteress. The punishment is recorded in number of traditions and the practice of the Prophet (peace and blessings be upon him) stands as an authentic source supporting it. This is the view held by all Companions, Successors and other Muslim scholars…”