The [unmarried] woman or [unmarried] man found guilty of sexual intercourse1 - lash each one of them with a hundred lashes,2 and do not be taken by pity for them in the religion [i.e., law] of Allāh,3 if you should believe in Allāh and the Last Day. And let a group of the believers witness their punishment.
Translation: Saheeh International (English)
1 Either by voluntary confession of the offender or the testimony of four male witnesses to having actually seen the act take place. Otherwise, there can be no conviction.
2 The ruling in this verse is applicable to unmarried fornicators. Execution by stoning is confirmed in the sunnah for convicted adulterers.
3 i.e., Do not let sympathy for a guilty person move you to alter anything ordained by Allāh, for in His law is protection of society as a whole.
The unmarried fornicating man and woman - flog each one of them with a hundred lashes, and no softness or mercy should affect you, whereby you do not establish the fixed punishment upon them or you make it lighter for them, if you believe in Allah and the Last Day. And a group of believers should be present when the fixed punishment is being executed on them, to go to the limit of publicising their crime, and to deter them and others.