Leviticus 5:6-7

6Then he is to bring his trespass offering to Adonai for his sin that he committed: a female from the flock, a lamb or a goat, as a sin offering. So the kohen is to make atonement for him over his sin.

7“But if one cannot afford a lamb, then he shall bring, as his trespass offering for the sin he committed, two turtledoves or two young pigeons, to Adonai. One is for a sin offering and the other for a burnt offering.