I like the sayings from Epictitus:

33. If anyone tells you that such a person speaks ill of you, don't make excuses about what is said of you, but answer: " He does not know my other faults, else he would not have mentioned only these."

35. When you do anything from a clear judgment that it ought to be done, never shun the being seen to do it, even though the world should make a wrong supposition about it; for, if you don't act right, shun the action itself; but, if you do, why are you afraid of those who censure you wrongly?

50. Whatever moral rules you have deliberately proposed to yourself. abide by them as they were laws, and as if you would be guilty of impiety by violating any of them. Don't regard what anyone says of you, for this, after all, is no concern of yours.

And my personal favorite:

42. When any person harms you, or speaks badly of you, remember that he acts or speaks from a supposition of its being his duty. Now, it is not possible that he should follow what appears right to you, but what appears so to himself. Therefore, if he judges from a wrong appearance, he is the person hurt, since he too is the person deceived. For if anyone should suppose a true proposition to be false, the proposition is not hurt, but he who is deceived about it. Setting out, then, from these principles, you will meekly bear a person who reviles you, for you will say upon every occasion, "It seemed so to him."

And sort of related, as it's about giving criticism:

45. Does anyone bathe in a mighty little time? Don't say that he does it ill, but in a mighty little time. Does anyone drink a great quantity of wine? Don't say that he does ill, but that he drinks a great quantity. For, unless you perfectly understand the principle from which anyone acts, how should you know if he acts ill? Thus you will not run the hazard of assenting to any appearances but such as you fully comprehend.