Hanash al-San'ani (may Allah have mercy on him) says: We went on a campaign to the Maghrib with Sayyiduna Ruwayfi' al-Ansari (may Allah be pleased with him). A town was conquered, and he stood up to deliver a sermon and said: I will say to you only what I heard from the Messenger of Allah (peace and blessings be upon him) at Khaybar. He (peace and blessings be upon him) said: "It is not lawful for a man who believes in Allah and the Last Day to water another's crop with his water, that is, to have intercourse with a pregnant woman (from the spoils). It is not lawful for a man who believes in Allah and the Last Day to have intercourse with a previously married captive woman until he has established that her womb is free. It is not lawful for a man who believes in Allah and the Last Day to sell any of the spoils of war before they are divided. It is not lawful for a man who believes in Allah and the Last Day to ride a mount from the fay' (property) of the Muslims until, when he has made it lean and weak, he returns it. And it is not lawful for a man who believes in Allah and the Last Day to wear a garment from the fay' of the Muslims until, when he has worn it out, he returns it."
Yunus bin Bukayr said 'the day of Khaybar' in this way, but it was only the day of Hunayn; others narrated it so from Ibn Ishaq, and others besides Ibn Ishaq narrated it so too. Others said 'Ruwayfi' bin Thabit', and that is correct.
Imam al-Shafi'i (may Allah have mercy on him) said: This indicates that captivity itself severs the marital bond between the spouses, for he (peace and blessings be upon him) would not order intercourse with a married woman after one menstrual period unless that had severed the bond. Ibn Mas'ud mentioned that the saying of Allah Almighty: "And (forbidden to you are) married women, except those whom your right hands possess" [Surah al-Nisa': 24] refers to "married women whom you have come to own by taking them captive."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18298