ROM 4:18

18 even though there was no physical reason for him to hope [that he would have descendants, because he and his wife were too old to bear children]. But God is the one who causes dead people to live again, and who talks about things that do not yet exist as already existing. [God said to Abraham], “You will have so many descendants that they will be [as impossible to count as the stars].” And Abraham believed that, and he believed that he would become the ancestor of many ethnic groups.