Allah said,

﴿وَالْمُحْصَنَـتُ مِنَ النِّسَآءِ إِلاَّ مَا مَلَكْتَ أَيْمَـنُكُمْ﴾

(Also (forbidden are) women already married, except those whom your right hands possess.) The Ayah means, you are prohibited from marrying women who are already married,

﴿إِلاَّ مَا مَلَكْتَ أَيْمَـنُكُمْ﴾

(except those whom your right hands possess) except those whom you acquire through war, for you are allowed such women after making sure they are not pregnant. Imam Ahmad recorded that Abu Sa`id Al-Khudri said, "We captured some women from the area of Awtas who were already married, and we disliked having sexual relations with them because they already had husbands. So, we asked the Prophet about this matter, and this Ayah was revealed, e

﴿وَالْمُحْصَنَـتُ مِنَ النِّسَآءِ إِلاَّ مَا مَلَكْتَ أَيْمَـنُكُمْ﴾

(Also (forbidden are) women already married, except those whom your right hands possess). Consequently, we had sexual relations with these women.'' This is the wording collected by At-Tirmidhi An-Nasa'i, Ibn Jarir and Muslim in his Sahih. Allah's statement,

﴿كِتَـبَ اللَّهِ عَلَيْكُمْ﴾

(Thus has Allah ordained for you) means, this prohibition was ordained for you by Allah. Therefore, adhere to Allah's Book, do not transgress His set limits, and adhere to His legislation and decrees.