It is narrated from Sayyiduna Salamah bin al-Muhabbiq (may Allah be pleased with him) that a man had intercourse with his wife's slave-girl, and the case was brought to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) said: "If she went along with him willingly, then she is his, and he owes (his wife) a slave-girl like her; and if he forced her, then the slave-girl is free, and he owes one like her," meaning that man will give his wife another slave-girl.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17072