The Prophet sal Allaahu alayhi wa sallam said:
“If someone curses you or tries to find fault with you, in a matter that does not exist in you,” or in other words he accuses you in a fault that you do not have, then, “do not find fault with him in a matter that he does have.”
So even if he lies about you and slanders you, do not point out the mistakes he himself has. He, sal Allaahu alayhi wa sallam, went on and said:
Leave him, because the evil of that deed will be against him, and the good will be for you.
In another place he said:
Do not ever curse any person.