Maliks Muwatta Book 29, Hadith Number 23.

Yahya related to me from Malik from Hisham ibn Urwa that he heard a man ask Urwa ibn az-Zubayr about a man who said to his wife, “Any woman I marry along with you as long as you live will be like my mother’s back to me.” Urwa ibn az-Zubayr said, “The freeing of slaves is enough to release him from that.”