Musannaf Ibn Abi Shayba 9:1005

Muhammad ibn al-Hasan narrated to us, from Sharik, from Salim, from Sa’id, regarding His saying: "And [also prohibited are] women already married, except those your right hands possess." He said: It was revealed concerning the women of the people of Hunayn. When the Messenger of Allah (peace be upon him) conquered Hunayn, the Muslims took captives. When a man wanted to have relations with one of them, she would say, "I have a husband." So they came to the Prophet (peace be upon him) and mentioned that to him. Then Allah the Exalted revealed: "And [also prohibited are] women already married, except those your right hands possess." He said: [This refers to] the captives who have husbands.