Musnad Ahmad 31:700

'Abd al-Razzaq narrated to us, Sufyan narrated to us, from 'Uthman al-Batti, from Abu al-Khalil, from Abu Sa'id al-Khudri, who said: We captured women from the captives of Awtas, and they had husbands. We disliked having intercourse with them while they had husbands, so we asked the Prophet, peace and blessings of Allah be upon him. Then this verse was revealed: "And [also prohibited are] women already married, except those your right hands possess" [An-Nisa: 24]. He said: "So we made their private parts lawful by it."