The Quran

وَالَّذينَ يَرمونَ المُحصَناتِ ثُمَّ لَم يَأتوا بِأَربَعَةِ شُهَداءَ فَاجلِدوهُم ثَمانينَ جَلدَةً وَلا تَقبَلوا لَهُم شَهادَةً أَبَدًا ۚ وَأُولٰئِكَ هُمُ الفاسِقونَ

Muhammad Asad

And as for those who accuse chaste women [of adultery],¹ and then are unable to produce four witnesses [in support of their accusation], flog them with eighty stripes;² and ever after refuse to accept from them any testimony - since it is they, they that are truly depraved!

The term muhsanat denotes literally "women who are fortified [against unchastity]", i.e., by marriage and/or faith and self-respect, implying that, from a legal point of view, every woman must be considered chaste unless a conclusive proof to the contrary is produced. (This passage relates to women other than the accuser's own wife, for in the latter case - as shown in verses 6-9 - the law of evidence and the consequences are different.
By obvious implication, this injunction applies also to cases where a woman accuses a man of illicit sexual intercourse, and is subsequently unable to prove her accusation legally. The severity of the punishment to be meted out in such cases, as well as the requirement of four witnesses - instead of the two that Islamic Law regards as sufficient in all other criminal and civil suits - is based on the imperative necessity of preventing slander and off-hand accusations. As laid down in several authentic sayings of the Prophet, the evidence of the four witnesses must be direct, and not merely circumstantial: in other words, it is not sufficient for them to have witnessed a situation which made it evident that sexual intercourse was taking or had taken place: they must have witnessed the sexual act as such, and must be able to prove this to the entire satisfaction of the judicial authority (Razi, summing up the views of the greatest exponents of Islamic Law). Since such a complete evidence is extremely difficult, if not impossible, to obtain, it is obvious that the purpose of the above Qur'anic injunction is to preclude, in practice, all third-party accusations relating to illicit sexual intercourse - for, "man has been created weak" (4: 28) - and to make a proof of adultery dependent on a voluntary, faith-inspired confession of the guilty parties themselves.