What is the ruling on swearing by divorce in this manner, and what is its expiation?

If a man says to his wife: "Do not go to your family's house, and if you do, I will divorce you," this is a promise of divorce, not an act of divorce or a contingent divorce. Therefore, no divorce occurs, and he may retract his statement. This is based on the saying of Ibn Taymiyyah: "A promise of divorce does not constitute divorce." However, if he swore by Allah concerning this, then he must pay expiation for an oath (kaffarat yamin) if his wife goes and he does not divorce her.