Shevuos 3:8-9

Shevuos 3:8

An oath taken in vain is if someone swore that a known thing was something else, such as that a stone pillar was made of gold, that a man was a woman, or that a woman was a man, or if one swore to something impossible, such as seeing a camel fly or seeing a snake the size of an olive-press beam. If a litigant called upon witnesses to testify on his behalf and they swore they would not do so, this oath is taken in vain (because it contradicts their Torah obligation). If one swore not keep a mitzvah, such as not to build a succah, wave a lulav, or put on tefillin, this is an oath taken in vain for which one is liable to lashes if it was taken intentionally and one is exempt if it was taken unintentionally.

Shevuos 3:9

If a person says, “I swear I will eat this loaf. I swear I will not eat it,” the first is a verbal oath and the second is an oath taken in vain. If he ate it, he is liable for making a vain oath and if he didn’t eat it, he is (also) liable for violating the verbal oath.