178. Amīr al-mu’minīn, peace be upon him, said: Cut away evil from the chest of others by snatching (it) away from your own chest. 
 This sentence can be interpreted in two ways. One is that if you bear malice against anyone, he too will bear malice against you. Therefore, destroy the malice from his heart by removing it from your heart, since your heart is the index of other’s heart. If your heart will have no malice there will remain no malice in his heart too. That is why a man assesses the purity of another person’s heart by the purity of his own heart. Thus, a man asked his friend, “How much do you love me?” and the reply was: “your own heart.” That is, “I love you as much as you love me.”
The second interpretation is that if you want to dissuade another person from evil, first you should refrain yourself from that evil. In this way, your advice can be effective on others, otherwise it will remain ineffective.