Al-Ma'idah 5:29

The real reward for the deeds performed for the sake of God will be given in the Hereafter. However, sometimes even in this world certain occurrences show whether human actions have gained God’s approval. This happened in the case of Adam’s sons, Abel and Cain (Habil and Qabil). Abel’s offerings from his flocks of sheep and goats were accepted by God and he was duly blessed, but Cain’s offerings of his agricultural produce were not, and he remained unblessed. Cain then became so jealous of his younger brother that he threatened to kill him. Abel said that Cain’s offering had been rejected because Cain had no heartfelt fear of God and should, therefore, reform himself instead of threatening him. But, jealousy and enmity blur the truth and one can think only of how to eliminate one’s supposed enemy. Abel told Cain that he might raise his hand to murder him, but that he would not retaliate, because God had prohibited any fighting between one Muslim and another. Even if a Muslim wanted to assassinate his brother, the brother should not consider it legitimate to kill his attacker. This obviated the mutual clash which was otherwise inevitable, as well as the resulting endless chain of action and reaction in Muslim society. If two Muslims are bent on each other’s destruction, the guilt is divided between the two. But if one Muslim carries out his intention, while the other remains immersed in prayer, the attacker will not only shoulder the burden of his own sin, but also that of any sin which his victim might have commmitted had he not remained patient and prayerful.