Musannaf Ibn Abi Shayba 7:255

Marwan ibn Mu'awiyah narrated to us, from Hassan ibn Abi Yahya, who said: I heard Ikrimah, and a man asked him, saying: "I swore to my wife by ten pilgrimages that if I had intercourse with a slave girl of mine [it would be such and such]." Ikrimah said: "If you had intercourse with her, she would belong to the Sultan. Go, for it is merely an oath, so offer expiation for it."