Leviticus 19:17

Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him.

Thou shalt in any wise rebuke thy neighbour. Instead of cherishing latent feelings of malice, or meditating purposes of revenge, against a person who has committed an insult or injury against them, God's people were taught to remonstrate with the offender, and endeavour, by calm and kindly reason, to bring him to a sense of taught to remonstrate with the offender, and endeavour, by calm and kindly reason, to bring him to a sense of his fault.

Not suffer sin upon him - literally, that ye may not participate in his sin.