What is the expiation for one who broke his oath after swearing not to eat in his house for two days, and then ate?

Breaking an oath may be obligatory if abstaining from food leads to harm to oneself or the body, or causes weakness in performing an obligation. This is preferable to persisting in abstaining from food, even if it does not affect one's health, because the Prophet, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do that and expiate for his oath." And she must offer expiation for the oath.