Siman 485

One who swears not to eat matzah on the night of Pesach—he is lashed and he eats matzah. For an oath does not take effect upon a matter of a mitzvah to nullify it, as he is already sworn and stands from Mount Sinai to fulfill the mitzvah, and he has sworn a vain oath and is lashed for it, as I have written in Yoreh Deah in siman 236. However, if he swore not to eat matzah the entire year or the entire month, where the oath takes effect upon the days before Pesach and those after it—it automatically takes effect also upon Pesach through inclusion, in the nullification of a mitzvah by "sit and do not act." And it appears to me that even if he swore regarding all the days of Pesach—it takes effect through inclusion, for since the oath takes effect regarding everything besides the first night, it automatically takes effect also upon it through inclusion.

But if he swore not to drink the four cups, which are of rabbinic origin, or regarding maror in this era, which is rabbinic, or regarding the second night, where even the eating of matzah is of rabbinic origin—his oath takes effect, for an oath takes effect upon the words of the scribes to nullify a mitzvah through "sit and do not act." Therefore, they shall compel him to release his oath through inquiry and regret, and to fulfill the commandment of the sages. And so too regarding an inclusive oath, they shall compel him to release the oath. And know that nedarim take effect even upon mitzvot, as I wrote in Yoreh Deah siman 215. Therefore, if he said, "The eating of matzah is konam upon me"—to forbid eating matzah. Therefore, one must compel him to release the neder through inquiry and regret, as I wrote there in siman 228. And the reason for the difference between a neder and a shevuah was explained there with the help of Heaven.