Surah 4. An-Nisaa', Ayah 156

The calumny referred to is the popular Jewish assertion that Jesus was an illegitimate child.

 i.e., they slandered Mary by claiming that she conceived Jesus through an illegal relationship.

The false charge against Mary was that she was unchaste. Cf. xix. 27-28. Such a charge is bad enough to make against any woman, but to make it against Mary, the mother of Jesus, was to bring into ridicule Allah's power itself. Islam is specially strong in guarding the reputation of women. Slanderers of women are bound to bring four witnesses in support of their accusations, and if they fail to produce four witnesses, they are to be flogged with eighty stripes and debarred from being competent witnesses: xxiv. 4.