Genesis 4:13-15

Cain said to the LORD, ‘My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.’ But the LORD said to him, ‘Not so; anyone who kills Cain will suffer vengeance seven times over.’ Then the LORD put a mark on Cain so that no-one who found him would kill him.