Genesis 4:5-8

but he did not accept Cain and his gift. This made Cain very angry, and he looked dejected. “Why are you so angry?” the LORD asked Cain. “Why do you look so dejected? You will be accepted if you do what is right. But if you refuse to do what is right, then watch out! Sin is crouching at the door, eager to control you. But you must subdue it and be its master.” One day Cain suggested to his brother, “Let’s go out into the fields.” And while they were in the field, Cain attacked his brother, Abel, and killed him.