“If anyone tells you that a certain person speaks ill of you, do not make excuses about what is said of you but answer, “He was ignorant of my other faults, else he would not have mentioned these alone.” 
― Epictetus

Epictetus was well known as a wiseass, and it’s hard not to love him for quotes like this.

More importantly, and these are my own words, be wary of informers. They are often motivated more by the need to feel valuable and ingratiate themself to you than they are to be of any real assistance.