Musannaf Ibn Abi Shayba 13:2291

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Haram ibn Uthman, from Abu Atiq, from Jabir, who said: The Prophet, peace and blessings of Allah be upon him, forbade the *kharaj* (earnings/tribute) of a slave woman, unless she is in constant labor.