Sunan an-Nasa'i 3333

It was narrated from Abu Sa'eed Al-Khudri that the Prophet of Allah sent an army to Awtas. They met the enemy, fought them, and prevailed over them. They acquired female prisoners who had husbands among the idolaters. The Muslims felt reluctant to be intimate with them. Then Allah, the Mighty and Sublime revealed:"Also (forbidden are) women already married, except those (slaves) whom your right hands possess," meaning, this is permissible for you once they have completed their 'Iddah