Muwatta Imam Malik Book of Marriage, Hadith 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."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ سَعِيدَ بْنَ الْمُسَيَّبِ، سُئِلَ عَنِ الْمَرْأَةِ، تَشْتَرِطُ عَلَى زَوْجِهَا أَنَّهُ لاَ يَخْرُجُ بِهَا مِنْ بَلَدِهَا فَقَالَ سَعِيدُ بْنُ الْمُسَيَّبِ يَخْرُجُ بِهَا إِنْ شَاءَ . قَالَ مَالِكٌ فَالأَمْرُ عِنْدَنَا ذَلِكَ أَنَّهُ إِذَا شَرَطَ الرَّجُلُ لِلْمَرْأَةِ وَإِنْ كَانَ ذَلِكَ عِنْدَ عُقْدَةِ النِّكَاحِ أَنْ لاَ أَنْكِحَ عَلَيْكِ وَلاَ أَتَسَرَّرَ إِنَّ ذَلِكَ لَيْسَ بِشَىْءٍ إِلاَّ أَنْ يَكُونَ فِي ذَلِكَ يَمِينٌ بِطَلاَقٍ أَوْ عِتَاقَةٍ فَيَجِبُ ذَلِكَ عَلَيْهِ وَيَلْزَمُهُ .