Genesis 4:5-6

4:5 but with Cain and his offering he was not pleased. So Cain became very angry, and his expression was downcast.

4:6 Then the Lord said to Cain, “Why are you angry, and why is your expression downcast?

Genesis 4:8

4:8 Cain said to his brother Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother Abel and killed him.