Buddhists must not harm themselves by harbouring the slightest negative thought. It may be fine if you bear a grudge against others for a month. However, if you bear it for a year, two years or even three years, it will only cause you serious harm.
学佛人不能把任何一点一滴不好的心态放在心中，去伤害自己。你把对别人的恨放在心中一个月没关系，放一年、两年、三年之后，你就会严重伤害到自己 。
– Master Jun Hong Lu, Words of Wisdom Volume 5
