Reference for Shevuot 53:1
<big><strong>מתני׳</strong></big> נשבע לבטל את המצוה ולא ביטל פטור לקיים ולא קיים פטור שהיה בדין שיהא חייב כדברי ר' יהודה בן בתירא
<br> MISHNAH. IF HE SWORE TO ANNUL A PRECEPT, AND DID NOT ANNUL IT, HE IS EXEMPT; TO FULFIL [A PRECEPT], AND DID NOT FULFIL IT, HE IS EXEMPT; THOUGH LOGICALLY [IN THE SECOND INSTANCE] HE SHOULD HAVE BEEN LIABLE, AS IS THE OPINION OF R. JUDAH B. BATHYRA: [FOR] R. JUDAH B. BATHYRA SAID: NOW, IF FOR AN OPTIONAL MATTER, FOR WHICH HE IS NOT ADJURED FROM MOUNT SINAI, HE IS LIABLE; FOR A PRECEPT, FOR WHICH HE IS ADJURED FROM MOUNT SINAI, HE SHOULD MOST CERTAINLY BE LIABLE! THEY REPLIED TO HIM: NO! IF YOU SAY THAT FOR AN OATH IN 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 FULFIL] A PRECEPT [HE IS LIABLE], SINCE [SCRIPTURE] HAS NOT IN THAT CASE MADE NEGATIVE EQUAL TO POSITIVE, FOR IF HE SWORE TO ANNUL [A PRECEPT], AND DID NOT ANNUL IT, HE IS EXEMPT! GEMARA. Our Rabbis taught: I might think that if he swore to annul a precept, and did not annul it, he should be liable, therefore it is said: to do evil, or to do good; just as doing good is optional, so doing evil must be optional; I must therefore exclude: if he swore to annul a precept, and did not annul it; for which he is exempt. I might think that if he swore to fulfil a precept, and did not fulfil it, he should be liable, therefore it is said: to do evil, or to do good; just as doing evil is optional, so doing good must be optional; I must therefore exclude: if he swore to fulfil a precept, and did not fulfil it; for which he is exempt. <br>