From The Heart

Words that emanate from the heart, penetrate the heart. דברים היוצאים מן הלב נכנסים ללב. There is no actual source in Chazal for this statement but there is an allusion in the pasuk.והיו הדברים  האלה אשר אנכי מצוך היום על לבבך - The words that I command you should be on your heart. Then the pasuk continues ושננתם לבניך - teach them to your children. If they are on your heart, you can properly teach them to your children.