Allah ta`ala says: "Do not come near Zina, for it is a shameful deed and an evil path." 17:32

"The male and female who commit fornication - flog each one of them with one hundred lashes, and do not let compassion for the two of them keep you from (complying with) Allah's religion, if you believe in Allah and the Last Day. And let a group of believers witness their punishment." [24:2]

The Jurists say that the last verse quoted above is the punishment for fornicators who have never been married. If a person was ever married before committing the crime of Zina, he or she is to be punished by stoning to death. This is established by the practise of the Prophet sallallahu `alayhi wa sallam. If they are not punished in this world and die without repentance (by regretting, leaving the sin, and so forth), they will be punished in the Fire with whips of fire.

In a tradition transmitted by Bukhari, on the authority of Samurah ibn Jundab (raa), concerning a dream of the Prophet in which the Prophet accompanied by Jibreel and Mikaail, the Prophet (saws) said: "We went on and arrived near a pit which was like an oven, out of which we could hear cries. We glanced into it and saw naked men and women, who cried out when the flames reached them from below. I asked, "Who are these, o Jibreel?" He replied: "Males and females who have committed fornication."

In explaining the meaning of the verse concerning Hell, "It has seven gates, to each of these gates a class (of sinners) is assigned." (15:44); `Ataa says: "The most severe of these gates in terms of heat, pain, sorrow, and stinking air is the gate for the doers of fornication, who did it (even) after knowing the awfulness of it." From Islaam.com