Reference for Shevuot 53:2
א"ר יהודה בן בתירא מה אם הרשות שאינו מושבע עליו מהר סיני הרי הוא חייב עליו מצוה שהוא מושבע עליה מהר סיני אינו דין שיהא חייב עליה
I might think that if he swore to do evil to himself, and did not do so, that he should be exempt, therefore it is said: to do evil, or to do good; just as doing good is optional, so doing evil must be optional; I will therefore include: if he swore to do evil to himself, and did not do so, [that he is liable,] for the option is in his own hands. I might think that if he swore to do evil to others, and did not do so, that 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 will therefore exclude: if he swore to do evil to others, and did not do so, [that he is exempt], for the option is not in his hands. Whence do we know to include [an oath] to do good to others? Because it is said: or to do good. And what is doing evil to others? T shall smite So-and-so, and crack his brain.'