This world is like a snowy mountain that echoes your voice. Whatever you speak, good or evil, will somehow come back to you. Therefore, if there is someone who harbors ill thoughts about you, saying similarly bad things about him will only make matters worse. You will be locked in a vicious circle of malevolent energy. Instead for forty days and nights say and think nice things about that person. Everything will be different at the end of forty days, because you will be different inside.

From The Forty Rules of Love by Elif Shafak