Tractate Shevuot - Chapter 3 - Mishnah 6 שבועות פרק ג׳ משנה ו׳

If he swore to annul a commandment, and did not annul it, he is exempt. [If he swore] to fulfill [a commandment], and did not fulfill it, he is exempt. For it would have been logical [in the second instance] that he should have been liable, as is the opinion of Rabbi Judah ben Bathyra. [For] Rabbi Judah ben Bathyra said, “Now, if for [swearing with regards to] an optional matter, for which he is not adjured from Mount Sinai, he is liable [should he not fulfill his oath], for [swearing with regards to] a commandment, for which he is adjured from Mount Sinai, he should most certainly be liable [should he not fulfill his oath]! They said to him: “No! If you say that for an oath with regards to an optional matter [he is liable], it is because [Scripture] has in that case made negative equal to positive [for liability]; But how can you say that for an oath [to fulfill] a commandment [he is liable], since [Scripture] has not in that case made negative equal to positive, for if he swore to annul [a commandment], and did not annul it, he is exempt!