Leviticus 5:4-6

4Or if anyone swears [an oath] thoughtlessly or impulsively aloud that he will do either evil or good, in whatever manner a person may speak thoughtlessly or impulsively with an oath, but he is unaware of it, when he recognizes it, he will be guilty in one of these.

5So it shall be when a person is guilty in one of these, that he shall confess the sin he has committed.

6He shall bring his guilt offering to the Lord for the sin which he has committed, a female from the flock, a lamb or a goat as a sin offering. So the priest shall make atonement on his behalf for his sin.