It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The umm walad (a slave woman who has borne her master's child) is free, even if the child was miscarried."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11559