Leviticus 5:1, 6

“‘When a person sins in that he hears a public curse against one who fails to testify and he is a witness (he either saw or knew what had happened) and he does not make it known, then he will bear his punishment for iniquity.

and 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.