Genesis 4:13-15
13 Cain said to the Lord, “My punishment is more than I can bear.
14 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.”
15 But the Lord said to him, “Not so; if anyone kills Cain, he will suffer vengeance seven times over.” Then the Lord put a mark on Cain so that no one who found him would kill him.