Love your enemies, and do not return evil with evil, but with goodness. Sound familiar? Here are some of the many sources for these widespread ethics...
Buddhism
"He abused me, he beat me, he defeated me, he robbed me!" In those who harbor such thoughts hatred is not appeased.
"He abused me, he beat me, he defeated me, he robbed me!" In those who do not harbor such thoughts hatred is appeased.
Hatreds never cease through hatred in this world; through love alone they cease. This is an eternal law." -- Dhammapada 3-5
"Conquer anger by love. Conquer evil by good. Conquer the stingy by giving. Conquer the liar by truth." -- Dhammapada 223
Christianity
"You have heard that it was said, 'You shall love your neighbor and hate your enemy.' But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven; for he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust. For if you love those who love you, what reward have you? Do not even the tax collectors do the same? And if you salute only your brethren, what more are you doing than others? Do not even the Gentiles do the same? You, therefore, must be perfect, as your heavenly Father is perfect." -- Matthew 5.43-48
"Do not be overcome by evil, but overcome evil with good." -- Romans 12.21
Confucianism
"Of the adage, Only a good man knows how to like people, knows how to dislike them, Confucius said, "He whose heart is in the smallest degree set upon Goodness will dislike no one." -- Analects 4.3-4
Islam
"It may be that God will ordain love between you and those whom you hold as enemies. For God has power over all things; and God is Oft-forgiving, Most Merciful." -- Qur'an 60.7
"The good deed and the evil deed are not alike. Repel the evil deed with one which is better, then lo!, he between whom and you there was enmity shall become as though he were a bosom friend. But none is granted it save those who are steadfast, and none is granted it save a person of great good fortune." -- Qur'an 41.34-35
Jainism
"My Lord! Others have fallen back in showing compassion to their benefactors as you have shown compassion even to your malefactors. All this is unparalleled." -- Vitaragastava 14.5
Sikhism
"What kind of love is this that to another can shift? Says Nanak, True lovers are those who are forever absorbed in the Beloved. Whoever discriminates between treatment held good or bad, Is not a true lover--he rather is caught in calculations." -- Adi Granth, Asa-ki-Var, M.2, p. 474
Taoism
"The sage has no fixed [personal] ideas.He regards the people's ideas as his own.I treat those who are good with goodness,And I also treat those who are not good with goodness.Thus goodness is attained. I am honest with those who are honest,And I am also honest with those who are dishonest.Thus honesty is attained." -- Tao Te Ching 49
"Do good to him who has done you an injury." -- Tao Te Ching 63