Leviticus 5:4

Or if a soul swear, pronouncing with his lips to do evil, or to do good, whatsoever it be that a man shall pronounce with an oath, and it be hid from him; when he knoweth of it, then he shall be guilty in one of these.

If a soul swear - a rash oath, without duly considering the nature and consequences of the oath, perhaps inconsiderately binding himself to do anything wrong, or neglecting to perform a vow to do something good. In all such cases a person might have transgressed one of the divine commandments unwittingly, and have been afterward brought to a sense of his delinquency.