Maliks Muwatta Book 28, Hadith Number 16.

Yahya related to me from Malik that he had heard that Said ibn al-Musayyab was asked about a woman who made a stipulation on her husband not to take her away from her town. Said ibn al-Musayyab said, “He takes her away if he wishes.”

Malik said, “The custom among us is that when a man marries a woman, and he makes a condition in the marriage contract that he will not marry after her or take a concubine, it means nothing unless there is an oath of divorce or setting-free attached to it. Then it is obliged and required of him.”