Genesis 15:1-6 (New Revised Standard Version)
After these things the word of the Lord came to Abram in a vision, "Do not be afraid, Abram, I am your shield; your reward shall be very great." But Abram said, "O Lord God, what will you give me, for I continue childless, and the heir of my house is Eliezer of Damascus?" And Abram said, "You have given me no offspring, and so a slave born in my house is to be my heir." But the word of the Lord came to him, "This man shall not be your heir; no one but your very own issue shall be your heir." He brought him outside and said, "Look toward heaven and count the stars, if you are able to count them." Then he said to him, "So shall your descendants be." And he believed the Lord; and the Lord reckoned it to him as righteousness.
Romans 4:1-5 (New Revised Standard Version)
What then are we to say was gained by Abraham, our ancestor according to the flesh? For if Abraham was justified by works, he has something to boast about, but not before God. For what does the scripture say? "Abraham believed God, and it was reckoned to him as righteousness." Now to one who works, wages are not reckoned as a gift but as something due. But to one who without works trusts him who justifies the ungodly, such faith is reckoned as righteousness.