Genesis 15:3-6

You have given me no descendants of my own, so one of my servants will be my heir.” Then the LORD said to him, “No, your servant will not be your heir, for you will have a son of your own who will be your heir.” Then the LORD took Abram outside and said to him, “Look up into the sky and count the stars if you can. That’s how many descendants you will have!” And Abram believed the LORD, and the LORD counted him as righteous because of his faith.