Siman 550

(107.) I heard in his name that he said: "When one repeats a falsehood twice — it becomes truth." Meaning: it becomes as truth in his eyes through repeating the false words twice. He also said there is one who lies in bed and fabricates from his heart some falsehood about his companion — that his companion said or did something improper to him, to anger him and the like. And he begins to grow angry about this — and his anger burns at his companion — and he becomes filled with rage toward him for nothing at all...