The Quran

الزّانِيَةُ وَالزّاني فَاجلِدوا كُلَّ واحِدٍ مِنهُما مِائَةَ جَلدَةٍ ۖ وَلا تَأخُذكُم بِهِما رَأفَةٌ في دينِ اللَّهِ إِن كُنتُم تُؤمِنونَ بِاللَّهِ وَاليَومِ الآخِرِ ۖ وَليَشهَد عَذابَهُما طائِفَةٌ مِنَ المُؤمِنينَ

Muhammad Asad

AS FOR the adulteress and the adulterer¹ - flog each of them with a hundred stripes, and let not compassion with them keep you from [carrying out] this law of God, if you [truly] believe in God and the Last Day; and let a group of the believers witness their chastisement.²

The term zina signifies voluntary sexual intercourse between a man and a woman not married to one another, irrespective of whether one or both of them are married to other persons or not: hence, it does not - in contrast with the usage prevalent in most Western languages - differentiate between the concepts of "adultery" (i.e., sexual intercourse of a married man with a woman other than his wife, or of a married woman with a man other than her husband) and "fornication" (i.e., sexual intercourse between two unmarried persons). For the sake of simplicity I am rendering zina throughout as "adultery", and the person guilty of it as "adulterer" or "adulteress", respectively.
The number of those to be present has been deliberately left unspecified, thus indicating that while the punishment must be given publicity, it need not be made a "public spectacle".