Muwatta Malik:Marriage:Hadith 1190
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ قَالَ الْمُحْصَنَاتُ مِنَ النِّسَاءِ هُنَّ أُولاَتُ الأَزْوَاجِ وَيَرْجِعُ ذَلِكَ إِلَى أَنَّ اللَّهَ حَرَّمَ الزِّنَا ‏.‏

Yahya related to me from Malik from Ibn Shihab that Said ibn al- Musayyab said, “The muhsanat among women are those who have husbands.” That referred to the fact that Allah has made fornication haram.