Qur'an (33:50) - "O Prophet! We have made lawful to thee thy wives to whom thou hast paid their dowers; and those (slaves) whom thy right hand possesses out of the prisoners of war whom Allah has assigned to thee"
Qur'an (23:5-6) - "..who abstain from sex, except with those joined to them in the marriage bond, or (the captives) whom their right hands possess..." This verse permits the slave-owner to have sex with his slaves. See also Qur'an (70:29-30).
Qur'an (4:24) - "And all married women (are forbidden unto you) save those (captives) whom your right hands possess." Even sex with married slaves is permissible.
Bukhari (62:137) - An account of women taken as slaves in battle by Muhammad's men after their husbands and fathers were killed. The woman were raped with Muhammad's approval
