Genesis 4:13-15

Cain said to YAHWEH, “My punishment is more than I can bear! Look—you’ve thrown me off the land today, and now I must hide from your presence. As a fugitive and wanderer on the earth, anyone who meets me may choose to kill me!” YAHWEH responded, “Not so! If anyone kills you, I promise the seven-fold vengeance of Cain will be released upon him!” So, YAHWEH put an identifying sign on Cain as a warning so that no one would dare kill him.