'והאמין בה AND HE BELIEVED IN THE LORD
— He did not ask Him for a sign regarding this; but in respect to the promise
that he would possess the land he asked for a sign, inquiring of God, במה אדע
“By what sign shall I know [that I shall possess it?]” (Genesis 15:8).
ויחשבה לו צדקה AND HE ACCOUNTED IT
UNTO HIM FOR RIGHTEOUSNESS — The Holy One, blessed be He, accounted it unto
Abraham as a merit, because of the faith with which he had trusted in Him.
Another explanation of במה אדע is: he did not, by these words, ask for a sign
regarding this promise that he would possess the land, but he said to Him,
“Tell me by what merit they (my descendants) will remain in it (the land).” God
answered him, through the merit of the sacrifices (Taanit 27b). (source)