Recite to them the truth of the story of the two sons of Adam. Behold! they each presented a sacrifice (to Allah): It was accepted from one, but not from the other. Said the latter: "Be sure I will slay thee." "Surely," said the former, "Allah doth accept of the sacrifice of those who are righteous. Al-Ma'idah 5:27 ⧉
Tafsir:
Whatever a man receives in this world is bestowed by God. Therefore, being jealous of a person on seeing him well situated in life and then inflicting harm on him amounts to trying to nullify God’s plan. Such a person is to some extent given the scope to do whatever he likes in this world of trial, but in the eyes of God, he is the worst sinner. Abel drew the attention of his elder brother, Cain, to this fact. This created some hesitation in his mind. He realised that he was bent on killing his younger brother without any justification. But the feeling of jealousy in him did not cool down. He invented excuses to justify his action. Ultimately, his inner struggle resolved itself and rest in his self-devised justification and he proceeded to kill his brother. The voice of conscience is the voice of God. A question arising in a man’s conscience about any action of his amounts in fact to his facing a test. A man is successful if he gives a positive response to the voice of his conscience, but if in taking the shelter of false excuses he suppresses the voice of his conscience, then he has failed.