Allah, the Wise, has said:
اَلزَّانِيَّةُ وَ الزَّانِـي فَاجْلِدُوا کُلُّ واَحِدٍ مِنْهُماَ مِائَةَ جَلدَةٍ
"The woman and the man guilty of adultery or fornication - flog each of them with a hundred stripes." 
The Noble Prophet (s.a.w) has said:
إِذاَ ظَهَرَ الزِّناَ مِنْ بَعْدِي کَثُرَ مَوْتُ الْفُجْأَةِ
"After my death, when fornication increases, sudden deaths shall (also) increase." 
Fornication is considered to be one of the Great Sins. A person, as a result of being overcome by lust and due to lack of faith, audaciously transgresses the womenfolk of other people.
In this world, fornication reduces sustenance and shortens the life-span, while on the Day of Judgment, a fornicator shall suffer the wrath of Allah and an evil reckoning.
The murderers of prophets, such as Prophet Yahya (a.s), and the auliya of Allah, such as Imam Husain (a.s), were mostly illegitimate children - a consequence of the Satan's invitation to a male and female to commit this sin.
Prophet Isa (a.s) said to his disciples: "Prophet Musa (a.s) had advised his people: "Do not commit fornication", (but) I advise you: "Do not even think of committing fornication""
1 – Five Fornicators and Five Rulings
Five persons, who had committed fornication, were brought before Umar, who ordered them to be subjected to the legal punishment for the crime. At that moment the Commander of the Faithful (a.s) arrived and said: "O' Umar! Allah's ruling in their case is not as you have ruled!" Umar requested: "You issue the ruling for them and implement the legal punishment."
The Imam (a.s) beheaded the first person, stoned the second, subjected the third to the full punishment of eighty lashes, the fourth to half the punishment and ordered the discretionary punishment for the fifth!
Umar was amazed and the people astounded over the Imam's (a.s) ruling. Umar asked him: "O' Abul Hasan! Five persons were involved in one crime and you have issued different rulings for all of them?"
the Commander of the Faithful(a.s) explained: "The first person was a dhimmi,) who had transgressed (the honour of) a Muslim woman and so had fallen out of the dhimmah) - as such, his punishment could only be death.
The second person who had committed fornication possessed a spouse and so I had him stoned. The third one to have been involved in the crime did not have a wife and so I punished him with eighty lashes.
The fourth was a slave and therefore I subjected him to half the legal punishment (of eighty lashes), while the fifth one was poor of intellect due to which I subjected him to the discretionary punishment (some lashes)."
Hearing the explanation Umar exclaimed: "O' Abul Hasan! May it never happen that I have to live with the people when you are not around (to help me)!"
2 – Interpretation of the Dream
Ibn Sirin was an extremely good-looking and elegant youth, and a cloth-merchant by profession. Once it so happened that a woman became enchanted with his looks; she expressed her desire to purchase cloth from him upon the condition that he should deliver it to her house, whereupon she would hand him the money.
As he reached her house and entered it, she immediately bolted the door and invited him to commit fornication. Aghast, the youth said: "I seek refuge in Allah", and then proceeded to lecture her about the reproach associated with the obscene act.
However when he observed that his words failed to have any effect upon the woman, he decided to resort to deception in order to extricate himself from his quandary.
He sought her permission to use the lavatory. Procuring her permission he entered the lavatory, dirtied himself with excrement and then approached her.
The woman, seeing him in that odious state, felt revolted and promptly turned him out of her house.
Due to this abstinence on his part with respect to fornication, Allah graced him with the science of interpretation of dreams. 
3 – Prophet Yahya’s Murderer was an Illegitimate Child
During the time of Prophet Yahya (a.s), there lived a king by the name of Hirodis, who was fond of him (a.s) and considered the Prophet to be a just and righteous person.
There was an adulteress with whom this king had been having an illicit relationship; as she became old, she began to ornament her daughter and present her before the king, who eventually fell in love with her and desired to marry her.
He questioned Prophet Yahya (a.s) about the issue who, on the basis of the religion of Prophet Isa (a.s), declared it to be unlawful. Consequently the adulteress harboured resentment and malice towards Prophet Yahya (a.s). One day, observing the king to be in a state of intoxication, she adorned her daughter and sent her before him. Seeing her, the king sought to gratify himself but the daughter said: "I shall submit myself to you only upon the condition that you sever Yahya's head from his body." The king, in his state of inebriation, agreed and upon his instance, Prophet Yahya (a.s) was beheaded.
According to another tradition, the king desired to enter into marriage with his niece, by the name of Hirodiya, but Prophet Yahya (a.s) prohibited such a marriage as a result of which the lady coveted the Prophet's death.
Imam Baqir (a.s) said: "Yahya's (a.s) killer was an illegitimate child, as were the killers of Ali and Husain Ibn Ali (a.s))."
When Prophet Yahya (a.s) was murdered Allah made Nebuchadnezzar (or Kardus), one of the Babylonian kings, dominant over Bait al-Maqdas, who initiated a massacre in the city. It was only when he had killed seventy thousand of the inhabitants of the city that Prophet Yahya's blood stopped bubbling and calmed down. 
4 – The Bath of Minjab
Once, a wealthy person, who was unmindful of Allah and always engaged in pleasure and enjoyment, happened to be sitting near the door of his house. A woman wanting to go to a bath, popularly known by the name of Minjab, had lost her way to it. Exhausted, she looked around if she could find someone who could guide her to the place when her eyes fell upon that man. Approaching him, she inquired: "Where is the Minjab bath?" Pointing to his house, he said: "This is the Minjab bath." Taking it to be the bath, she entered the house and the man quickly entered into it behind her. Once inside, he bolted the door and approaching the lady, sought to commit adultery.
The woman, perceiving that she had walked into the clutches of a immoral person and realizing that deception was the only means of escape, said to him: "I am ardently desirous too; however I am hungry and covered with dirt. Bring some perfume and some food; we can have food together after which I shall place myself at your disposal."
The man agreed and went out of the house. Having procured some food and perfume he returned home only to find the lady missing. Her escape upset him immensely and the craving for committing adultery with the lady remained so deeply embedded in his heart that he would always recite the following verses:
ياَ رُبَّ قاَئِلَةٍ يَوماً وَ قَد تعبتْ أَينَ الطَّرِيقُ إِلـى حَمَّامِ مَنْجَابِ
"What happened to the lady, who had become exhausted? And had asked: Where is the way to the Minjab bath?"
The time passed until it so happened that one day he lay on his death-bed. His acquaintances approached him and asked him to recite La Ilaha Illallah, Muhammadun Rasulullah, but instead, he would only recite the above verses - and eventually passed away in that very state.
5 – The Noble Prophet and the Youth
One day a youth approached the Noble Prophet (s.a.w) and with utmost disrespect and discourtesy, said: "O' Prophet of Allah! Do you permit me to commit fornication?"
These words created a great uproar amongst the people and objections could be heard from every nook and corner. The Noble Prophet (s.a.w), exhibiting sublime ethics and speaking with absolute gentleness, said: "Come near." The youth stepped forward and sat down beside him (s.a.w), whereupon the Prophet asked: "Would you like someone to commit the act with your mother?" The youth replied: "May I be made your ransom! Never." The Noble Prophet (s.a.w) said: "Similarly the (other) people too do not approve of this with respect to their daughters."
The Noble Prophet (s.a.w) continued: "Tell me: Would you approve of this act for your sister?" Once again the youth denied (and repented having asked the question).
The Noble Prophet (s.a.w) placed his hands upon the youth's chest and prayed for him, saying: "O' Lord! Purify his heart, forgive his sins and protect him from the pollution of depravity." From then onwards fornication became the worst of the deeds in the eyes of the youth.
 Suratul Nur (24), Verse 2
 Tafsir Mu~in, pg. 366
 Shaytan, vol. 1, pg. 560
 A fri non-Muslim subject living in a Muslim country. (Tr.
 Covenant of protection. (Tr.
 Qadhawat-ha-e-Muhayyir al-~Uqul, pg. 45; Dastan-ha-e-Zindagi-e-~Ali (a.s), pg. 145
 Shaytan, vol. 1, pg. 678
 Tarikh-e-Anbiya, vol. 2, pg. 284
 Alam-e-Barzakh, pg. 41; Kashkul (of Sheikh Bahai), vol. 1, pg. 232
 Dastan-ha Wa Pand-ha, vol. 3, pg. 138; Tafsir al-Manar, under the discussion related to verse 104 of SuratAle ~Imran