Leviticus 5:4-8

Or if a person swears thoughtlessly with his lips to do evil or to do good, in whatever matter people speak thoughtlessly with an oath, and it is hidden from him, and then he comes to know it, he will be guilty of one of these things.

So it shall be when he becomes guilty of one of these things, that he shall confess that in which he has sinned.

He shall also bring his guilt offering to the Lord for his 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.

‘But if he cannot afford a lamb, then he shall bring to the Lord his guilt offering for that in which he has sinned, two turtledoves or two young doves, one as a sin offering and the other as a burnt offering.

He shall bring them to the priest, who shall first offer that which is for the sin offering, and shall pinch off its head at the front of its neck, but he shall not sever it.