Musannaf Ibn Abi Shayba 9:307

Ali ibn Mushir narrated to us, from Isma'il ibn Abi Khalid, from al-Sha'bi, who said: "Whichever slave comes to a free woman and she marries him, and she learns of that [his status], he said: If she wishes, she may remain with him, and if she wishes, she may separate from him."