Isaiah 40:28-31 (ESV)

28 Have you not known? Have you not heard?
     The Lord is the everlasting God,
        the Creator of the ends of the earth.
     He does not faint or grow weary;
        his understanding is unsearchable.
29 He gives power to the faint,
        and to him who has no might he increases strength.
30 Even youths shall faint and be weary,
        and young men shall fall exhausted;
31 but they who wait for the Lord shall renew their strength;
        they shall mount up with wings like eagles;
     they shall run and not be weary;
        they shall walk and not faint.