God promises Abraham that despite his advanced age, he will have innumerable descendants. God shows him the stars, and tells him: “So shall be your descendants.” The Sages explain the precise meaning of this expression.

What is the meaning of the verse: “So shall be your descendants” (Genesis 15:5)? Rabbi Levi said in the name of Rabbi Yohanan: To what is the matter comparable? It is comparable to one who set out on the way and he went one, two, three, up to ten days, and he did not find a city, an inn, a tree, water, or a person. After walking for ten days, he saw one tree from a distance. He said: Perhaps there is water beneath it. Once he approached the tree, he found it standing beside a spring. Once he saw that the tree was beautiful, its fruits were tasty, and its branches were beautiful, he sat and cooled himself in its shade. He ate some of its fruits, drank water from the spring; and it was pleasant for him and his soul was restored. When he stood to go, he said to the tree: How can I bless you? What can I say to you? Shall I say that your wood should be beautiful? It is already beautiful. Shall I say that your shade shall be pleasant? It is already pleasant. Shall I say that your branches should be pleasing? They are already pleasing. Shall I say that your fruits shall be sweet? They are already sweet. Shall I say that a spring shall emerge from beneath your roots? A spring already emerges from beneath your roots. Shall I say that you shall stand in a charming place? You already stand in a charming place. How can I bless you? I bless you that all the saplings that come from you shall be like you. So too, when the Holy One, blessed be He, created the world, twenty generations stood and they served no purpose, and no righteous person stood among them until after twenty generations, [when Abraham was born]. The Holy One, blessed be He, said to him: Abraham! What can I say to you and how can I bless you? Shall I say that you shall be completely righteous before Me, or that your wife Sarah shall be righteous before Me? You are righteous and your wife Sarah is righteous before Me. Or shall I say that all the members of your household shall be righteous? They are all righteous before Me. How can I bless you? I bless you that all the children that are destined to come from you shall be like you. From where is that derived? As it is stated: “He said to him: So, [i.e., like you] shall be your descendants” (15:5). (Bemidbar Rabba 2)