Leviticus 5:7, 9, 10

“ ‘Anyone who cannot afford a lamb is to bring two doves or two young pigeons to the LORD as a penalty for their sin—one for a sin offering and the other for a burnt offering.

and is to splash some of the blood of the sin offering against the side of the altar; the rest of the blood must be drained out at the base of the altar. It is a sin offering.

The priest shall then offer the other as a burnt offering in the prescribed way and make atonement for them for the sin they have committed, and they will be forgiven.