An-Noor 24:2

As for the woman and the man who commit fornication; flog each of them a hundred lashes. Do not let pity for them make you lenient concerning the law of Allah, if you believe in Allah and the Last Day. And let a group of the believers witness their punishment[1].

[1] Fornication has to be proven either by confession or the testimony of four reliable witnesses to having actually seen the act take place. This ruling is applicable to unmarried fornicators. Execution by stoning is confirmed in the sunnah for married adulterers.

The [unmarried] woman or [unmarried] man found guilty of sexual intercourse[972] - lash each one of them with a hundred lashes,[973] and do not be taken by pity for them in the religion [i.e., law] of Allāh,[974] if you should believe in Allāh and the Last Day. And let a group of the believers witness their punishment.

[972] 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.
[973] The ruling in this verse is applicable to unmarried fornicators. Execution by stoning is confirmed in the sunnah for convicted adulterers.
[974] 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.

2. The fornicatress and the fornicator, flog each of them with a hundred stripes. Let not pity withhold you in their case, in a punishment prescribed by Allâh, if you believe in Allâh and the Last Day. And let a party of the believers witness their punishment. (This punishment is for unmarried persons guilty of the above crime, but if married persons commit it (illegal sex), the punishment is to stone them to death, according to Allâh’s Law).[1]

[1] (V.24:2)
a) Narrated Abu Hurairah رضي الله عنه: Allâh’s Messenger صلى الله عليه وسلم judged that the unmarried person who was guilty of illegal sexual intercourse be exiled for one year and receive the legal punishment (i.e., be flogged with one hundred stripes). (Sahih Al-Bukhâri, Vol.8, Hadith No. 819)
b) Narrated Jâbir bin ‘Abdullâh Al-Ansâri رضي الله عنهما: A man from the tribe of Bani Aslam came to Allâh’s Messenger صلى الله عليه وسلم and informed him that he had committed illegal sexual intercourse and he bore witness four times against himself. Allâh’s Messenger صلى الله عليه وسلم ordered him to be stoned to death as he was a married person. (Sahih Al Bukhari, Vol.8, Hadith No.805).
c) See (V.4:25).

The adulterer and the adulteress, scourge ye each one of them (with) a hundred stripes. And let not pity for the twain withhold you from obedience to Allah, if ye believe in Allah and the Last Day. And let a party of believers witness their punishment.