He asks a sign, not out of distrust of God's promise, for he was strong in faith, Romans 4:20, but for further assurance and confirmation of it. And such an asking of a sign was not an unusual practice with good men, as Judges 6:37 2 Kings 20:8, not are they reproved for it; but on the contrary, Ahaz was commanded to ask a sign, and reproved for not asking it, Isaiah 7:1-25.
Related Commentaries of Genesis 15:8
Genesis 15:8
8 And he said, Lord GOD, whereby shall I know that I shall inherit it?