Genesis 4:3-5, 15, 25

And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto Jehovah. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And Jehovah had respect unto Abel and to his offering: but unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.

And Jehovah said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And Jehovah appointed a sign for Cain, lest any finding him should smite him.

And Adam knew his wife again; and she bare a son, and called his name Seth: For, said she, God hath appointed me another seed instead of Abel; for Cain slew him.