Musannaf Ibn Abi Shayba 9:994

Ghundar narrated to us, from Ibn Jurayj, from ‘Ata’, who said: "It is adultery." And Mujahid said: "It is adultery." And ‘Ikrimah said: "It is adultery, except for those whom your right hands possess." And Ibn ‘Abbas said regarding "except for those whom your right hands possess": "A man takes the slave girl of his slave’s wife." And another said: "The captives of the enemy are to be had intercourse with when their husbands are captured."