The adulterer and the adulteress — flog each one of them with a hundred stripes…
-(An-Nur 24:3)

…And those who calumniate chaste woman but bring not four witnesses — flog them eighty stripes and do not admit their evidence ever after.
-(An-Nur 24:5)

In these verses, it is made clear that four witnesses are required to prove adultery — and not when the husband is being accused of unfaithfulness, but when the wife is being accused. This onus is meant to protect women from the evils of men who would destroy a woman’s reputation and name out of spite. If their accusation is honest, they are required to produce four impeccable witnesses to back up their claims. If they cannot, the accusers are to be punished severely for falsely accusing a woman and attempting to ruin her, and they are never to be trusted to give evidence again.