Leviticus 5:4-6

4or when a person swears an oath, speaking thoughtlessly with his lips, whether to do evil or to do good, with regard to anything which the individual might speak thoughtlessly in an oath, even if he did not realize it, but he himself has later come to know it and is guilty with regard to one of these oaths –

5when an individual becomes guilty with regard to one of these things he must confess how he has sinned,

6and he must bring his penalty for guilt to the Lord for his sin that he has committed, a female from the flock, whether a female sheep or a female goat, for a sin offering. So the priest will make atonement on his behalf for his sin.