[If] one swore to annul a mitzvah, and did not annul it —- he is exempt; to fulfill [a mitzvah], and he did not fulfill [it] —- he is exempt. For it might have been inferred that he be liable, as is the opinion of R' Yehudah ben Beseira. Said R' Yehudah ben Beseira: If for an optional matter, concerning which he was not adjured from Mount Sinai, he is liable; [then] for a mitzvah, concerning which he is adjured from Mount Sinai, is it not certain that he should be liable? They said to him: No! If you say this as regards an optional oath, it is because [Scripture] has made negative equal to positive in that regard. Will you say this in the case of an oath to fulfill a mitzvah, in which [Scripture] did not make negative equal to positive? For if he swears to annul it and does not annul it, he is exempt.