Musannaf Ibn Abi Shayba 20:34

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Ibn Abi Dhi’b narrated to us, from Makhlad ibn Khuffaf ibn Ima’ ibn Rakhada al-Ghifari, from ‘Urwah ibn al-Zubayr, from ‘A’ishah, she said: The Messenger of Allah, may Allah bless him and grant him peace, decreed that the earnings of a slave are in return for his liability.