The twenty-third of the wisdom sayings from Chapter Four of St Benedict's Rule is, not to nurse a grudge.
Its Scriptural basis is St Matthew 5:22-26:
"But I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, `You fool!' shall be liable to the hell of fire. So if you are offering your gift at the altar, and there remember that your brother has something against you, leave your gift there before the altar and go; first be reconciled to your brother, and then come and offer your gift. Make friends quickly with your accuser, while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison; truly, I say to you, you will never get out till you have paid the last penny."