Maliks Muwatta Book 31, Hadith Number 5.
Section : What is Done about Slave-Girls when Purchased and Conditions Made about Them.
Yahya related to me from Malik from Ibn Shihab that Ubaydullah ibn Abdullah ibn Utba ibn Masud told him that Abdullah ibn Masud bought a slave-girl from his wife, Zaynab Ath Thaqafiyya. She made a condition to him, that if he bought her, she could always buy her back for the price that he paid. Abdullah ibn Masud asked Umar ibn al-Khattab about that and Umar ibn al-Khattab said, “Do not go near her while anyone has a condition concerning her over you.”