Leviticus 5:4-6

4Or if a person 1swears thoughtlessly with his lips to do evil or to do good, in whatever matter a man may speak thoughtlessly with an oath, and it is hidden from him, and then he comes to know it, he will be guilty in one of these. 5So it shall be when he becomes guilty in one of these, that he shall 2confess that in which he has sinned. 6He shall also bring his guilt offering to the Lord for his sin which he has 3committed, 4a female from the flock, a lamb or a 5goat as a sin offering. So the priest shall make atonement on his behalf for his sin.