“And those who invoke not any other God along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide there in disgrace. Except those who repent and believe and do righteous deeds.” (Quran: Al-Furqan 25:68)
“And come not near to the unlawful sexual intercourse. Verily, it is a transgression of Allah’s limits, and an evil way.” (Quran: Al-Israa 17:32)
“The adulterer marries not but an adulteress or a Mushrikah (polytheist woman) and the adulteress none marries her except an adulterer or a Mushrik (polytheist man). Such a thing is forbidden to the believers.” (Quran: An-Noor 24:3)
The Prophet Muhammad once talked about when he was taken by the Angel Gabriel to see the hellfire and its inhabitants. He narrated to his companions, “…and so we set out and came to something like a kiln narrow at the top and wide at the bottom, there came from it sounds and voices. He said, ‘We looked inside, and there were naked men and women and there would rise up fire from beneath them and when it reached them, they screamed.’ I said, ‘Who are these people, O Jibrail?’ He said, ‘These are the fornicators.'”